Convert Rust printing to value-based API

Convert Rust printing to value-based API

For Rust, it was simple to convert the printing code to the
value-based API all at once.

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

	* rust-lang.c (val_print_struct, rust_print_enum): Use the value
	API.
	(rust_val_print): Rewrite.
	(rust_value_print_inner): New function, from rust_val_print.
	(rust_language_defn): Use rust_value_print_inner.