Use bool in Rust code

This changes various functions in the Rust code to use a bool rather
than an int when a boolean is intended.

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

	* rust-exp.y (ends_raw_string, space_then_number)
	(rust_identifier_start_p): Return bool.
	* rust-lang.c (rust_tuple_type_p, rust_underscore_fields)
	(rust_tuple_struct_type_p, rust_tuple_variant_type_p)
	(rust_slice_type_p, rust_range_type_p, rust_u8_type_p)
	(rust_chartype_p): Return bool.
	(val_print_struct, rust_print_struct_def, rust_print_type):
	Update.
	* rust-lang.h (rust_tuple_type_p, rust_tuple_struct_type_p):
	Return bool.