More verbose -print-allocs messages. Initial commit.

[ebak]

This change intends to provide more verbose -print-allocs messages by naming the escaping intermediate variable or type.
Currently it is tested only with the unit tests.

Here are the modified messages, generated by allocs_test.go:

makeslice.buf size is not constant
makeslice4 size 300 exceeds maximum stack allocation size 256
makeslice6 escapes at line 30
FloatType escapes at line 37
varargs12 escapes at line 44
slicelit14 escapes at line 47
n4 escapes at line 51
n5 escapes at line 51
makeslice escapes at line 108
b escapes at line 117
Arg 0 of main.scaleVector3() call escapes at line 122
b escapes at unknown line

[dgryski]

Seems good but someone else a bit more familiar with this code should take a look.