Use std::string in Rust code

This changes a couple of spots in the Rust support to use std::string.
In one spot this removes some manual memory management; in the other
spot this allows the removal of a call to xstrdup.

2017-02-02  Tom Tromey  <tom@tromey.com>

	* rust-lang.h (rust_crate_for_block): Update.
	* rust-lang.c (rust_crate_for_block): Return std::string.
	(rust_get_disr_info): Use std:;string, not
	gdb::unique_xmalloc_ptr.
	* rust-exp.y (crate_name): Update.