"type mod" refers to agglutinative expressions such as dereference
"*", invocation "()", and slicing "[:]". When considering an object as
a completion candidate, we check whether applying a type mod would
make it a better candidate.

Previously we tracked the type mods we wanted to apply to a candidate
by setting bool fields. Now instead we keep a slice of the type mods.
This has two main advantages:
- The mods are now ordered which will allow us to format candidates
  properly when the same mods can appear in different order (e.g.
  "<-*foo" or *<-foo").
- We can now record any mod multiple times allowing for "<-<-foo" or
  "foo()()".

I changed the formatting code to always create a snippet object since
that made things simpler. I had to tweak a few snippet helper methods
to accept a snippet argument rather than creating a new snippet.

This commit's only functional change is that we no longer show any
type mods in candidate labels. For example, the user will now see
"foo" in the completion popup instead of "*foo". Showing the operators
adds noise to the candidate list, and we didn't display them
consistently.

Updates golang/go#46045.

Change-Id: I3ea7baa1ee2fee80a1f8cfe88cbae1093ae269ba
Reviewed-on: https://go-review.googlesource.com/c/tools/+/323449
Run-TryBot: Muir Manders <muir@mnd.rs>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Rebecca Stambler <rstambler@golang.org>

Now we search up to three levels for candidate modifiers. For example,
we can now complete "foo" to "foo()()" (double invocation).

Granted this is rarely useful, but it generalizes and simplifies the
searching we did for dereference modifiers.

Updates golang/go#46045.

Change-Id: Ibf0be8158e16a0a26a6344a346f34af8fe182bb0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/323450
Run-TryBot: Muir Manders <muir@mnd.rs>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Rebecca Stambler <rstambler@golang.org>