Reopening for minux to add more tests.

I wrote a trivial extension of your code. it works, but 6g takes a huge
amount
of memory to compile the resulting program.

6.92user 0.47system 0:07.38elapsed 100%CPU (0avgtext+0avgdata
1916388maxresident)k
0inputs+89560outputs (0major+483391minor)pagefaults 0swaps

I'm using the multiple init() trick, but it seems that is not enough.

Some interesting statistics:

1. all in the same function:
   7.28user 0.55system 0:07.83elapsed, 1965620K MSS
2. each typecast in a different init():
   7.30user 0.59system 0:07.89elapsed, 1900004K MSS
3. each test type (unary, binary, shift) for each type cast in different
   init():
   6.92user 0.47system 0:07.38elapsed, 1916388K MSS

Because gc always keeps every toplevel function in memory, so splitting the
functions
will not help reducing memory requirements much.

We still need to split the test into smaller pieces, like the rotate tests.

Too late for Go 1.5.