Convert Modula-2 printing to value-based API

Convert Modula-2 printing to value-based API

This finishes the conversion of Modula-2 printing to the value-based
API.  It does so by copying the body of m2_val_print into
m2_value_print_inner, and then introducing new functions as needed to
use the value API.

The "val_" API code continues to exist, because it's still possible
for it to be called via some paths.  This code will all be removed at
the end of the series.

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

	* m2-valprint.c (m2_print_unbounded_array): New overload.
	(m2_print_unbounded_array): Update.
	(m2_print_array_contents): Take a struct value.
	(m2_value_print_inner): Rewrite.