gccrs: improve mutability checks

gccrs: improve mutability checks

This ensures that we handle var decls readonly checks much better

Addresses: Rust-GCC#807 Rust-GCC#3287

gcc/rust/ChangeLog:

	* checks/errors/rust-readonly-check.cc (check_decl): improve mut check
	(emit_error): helper
	(check_modify_expr): likewise
	(readonly_walk_fn): reuse helper
	(ReadonlyCheck::Lint): cleanup context each run

gcc/testsuite/ChangeLog:

	* rust/execute/torture/builtin_macro_include_bytes.rs: needs mut
	* rust/compile/mutability_checks1.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>