Convert Go printing to value-based API

Convert Go printing to value-based API

This introduces go_value_print_inner, a modified copy of go_val_print.
Unlike some of the other languages, Go was straightforward to convert
to the value-based API all at once, so this patch takes that approach.

gdb/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* go-valprint.c (go_value_print_inner): New function.
	* go-lang.h (go_value_print_inner): Declare.
	* go-lang.c (go_language_defn): Use go_value_print_inner.