Fix &str printing in Rust

Fix &str printing in Rust

Printing a string slice ("&str") in Rust would print until the
terminating \0; but that is incorrect because a slice has a length.
This fixes &str printing, and arranges to preserve the type name when
slicing a slice, so that printing a slice of an "&str" works as well.

This is PR rust/22236.

2017-10-02  Tom Tromey  <tom@tromey.com>

	PR rust/22236:
	* rust-lang.c (rust_val_print_str): New function.
	(val_print_struct): Call it.
	(rust_subscript): Preserve name of slice type.

2017-10-02  Tom Tromey  <tom@tromey.com>

	PR rust/22236:
	* gdb.rust/simple.rs (main): New variable "fslice".
	* gdb.rust/simple.exp: Add slice tests.  Update string tests.