Use std::string, std::vector in rust-lang.c

Use std::string, std::vector in rust-lang.c

This patch changes some spots in rust-lang.c to use std::string or
std::vector, removing some cleanups.

2016-09-23  Tom Tromey  <tom@tromey.com>

	* rust-lang.c: Include <string> and <vector>.
	(rust_evaluate_funcall): Use std::vector, std::string.
	(rust_evaluate_subexp): Use std::string.
	(rust_lookup_symbol_nonlocal): Use std::string.