New behavior is turned on by a "/<" cs-mark presence
in a first comment of the block.

The tsPos and csPos below stand for type-start and comment-start
positions, respectively. cmPos stands for cs-mark position.

Tabwidth is fixed at 8. Measurements and adjusts are relative to the
block content start position, ie. are indent-independent.

NONE heuristics are used, gofmt just obey the recipe as set here.

Recipe:

R1. Set tsPos to the tabstop that follows the opening brace position.
R2. if cmPos <= tsPos {; cmPos += 8 }     // make sure cmPos > tsPos 
R3. If cmPos is not at tabstop now, advance it to the next tabstop.
R4. Set cs-mark position from the current (adjusted) cmPos.
R5. Rewrite cs-mark line.
R6. Adjust lines in block, shifting relevant parts using spaces.
R7. If text overflows one or the other "start" position, use single space.
    Human can react to spurious overflows moving comment-mark a space or two.
R8. Estabilished tsPos and csPos are valid to the end of block.
    Open question: should we recognize cs-mark in the inner scopes?


On Examples:

// Youri uses tw=8 and writes:
type Other struct { //      /< author set marker here
        counter int // midlen 
        nce int64 // short
        longlongname string // long
}

// e Other struct { //      /< author set comment-start position
//      |---------#-----|         R1 type starts at next ts
//      |-------------------|        given comment-start position
//      |-------------------#---| R3 adjusted comment-start position
//......A.......B.......C.......D.......E.......F.......G.......H...
// 
// Now Youri sees:
type Other struct { //          /< adjusted cs-mark
        counter         int     // midlen 
        nce             int64   // short
        longlongname    string  // long
}

// Bartek tw=4 sees:
type Other struct { //          /< adjusted cs-mark
    counter         int     // midlen 
    nce             int64   // short
    longlongname    string  // long
}

// Beata tw=2 sees:
type Other struct { //          /< adjusted cs-mark
  counter         int     // midlen 
  nce             int64   // short
  longlongname    string  // long
}


// Now Beata writes, she is using tw=2
type Other struct { // /< just near longlongname, as perceived
  counter int // midlen 
  nce int64 // short
  longlongname string // long
}
// e Other struct { // /< author set comment-start position
//      |---------#-----|         R1 type starts at next ts
//      |--------------|             given comment-start position
//      |----------------------|  R2 +8 shift 
//      |--------------#--------| R3 adjusted comment-start position
//......A.......B.......C.......D.......E.......F.......G.......H...
//
// Beata ts=2 now sees:
type Other struct { //          /< adjusted
  counter         int     // midlen 
  nce             int64   // short
  longlongname    string  // long
}
// Bartek ts=4 sees:
type Other struct { //          /< adjusted
    counter         int     // midlen 
    nce             int64   // short
    longlongname    string  // long
}
// Youri ts=8 sees:
type Other struct { //          /< adjusted
        counter         int     // midlen 
        nce             int64   // short
        longlongname    string  // long
}

//
// Excercise R7 rule.
//
// Beata ts=2 writes:
type sn struct { //   /< Beata manually adjusted to longlongname
  field int // midlen 
  n   int64 // short
  longlongname string // r
}
// e sn struct { //   /< Beata manually adjusted to longlongname
//      |------#|         R1 type starts at next ts
//      |-------------|      given comment-start position
//      |-------------#-| R3 adjusted comment-start position
//......A.......B.......C.......D.......E.......F.......G.......H...
//
// Beata ts=2 now sees:
type sn struct { //     /< adjusted 
  field   int     // midlen 
  n       int64   // short
  longlongname string // R7
}

// Bartek ts=4 sees:
type sn struct { //     /< adjusted 
    field   int     // midlen 
    n       int64   // short
    longlongname string // R7
}

// Youri ts=8 sees:
type sn struct { //     /< adjusted 
        field   int     // midlen 
        n       int64   // short
        longlongname string // R7
}

// Beata moved mark by three spaces right:
// 
// Beata ts=2 writes:
type sn struct { //      /< three spaces more
  field int // midlen 
  n   int64 // short
  longlongname string // r
}
// e sn struct { //      /< three spaces more 
//      |------#|                 R1 type starts at next ts
//      |----------------|           given comment-start position
//      |----------------#------| R3 adjusted comment-start position
//......A.......B.......C.......D.......E.......F.......G.......H...
// tw=2
type sn struct { //             /< adjusted
  field   int             // midlen 
  n       int64           // short
  longlongname string     // R7 for type
}
// tw=4
type sn struct { //             /< adjusted
    field   int             // midlen 
    n       int64           // short
    longlongname string     // R7 for type
}
// tw=8
type sn struct { //             /< adjusted
        field   int             // midlen 
        n       int64           // short
        longlongname string     // R7 for type
}