The argument context string is only used in error messages. Don't format
the function AST into a string for every single argument of every single
call that is type-checked. Instead do it once per call (still not great,
but much much better).

Performance optimization.

Change-Id: Iec87f9ad34128d7b3eee58577ad37dbaa8e6db44
Reviewed-on: https://go-review.googlesource.com/c/157037
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>