Added FFIVector to get Polonius output on C++ side

Added FFIVector to get Polonius output on C++ side

ChangeLog:

	* .github/workflows/ccpp.yml: Switch to nightly version of rustc
	to use unstable features, namely extern types for FFI.

gcc/rust/ChangeLog:

	* Make-lang.in: Compile new file, rust-polonius.cc
	* checks/errors/borrowck/ffi-polonius/src/gccrs_ffi.rs: Opaque
	type to represent FFIVector from C++.
	* checks/errors/borrowck/ffi-polonius/src/gccrs_ffi_generated.rs:
	Change types of fields in Output.
	* checks/errors/borrowck/ffi-polonius/src/lib.rs: Added helper
	functions to contruct Polonius output on C++ side,
	used helpers to contruct Polonius output on C++ side.
	* checks/errors/borrowck/polonius/rust-polonius-ffi.h (make_vector):
	FFIVector is a wrapper around std::vector for transfering data
	from Rust to C++.
	(struct Output): Use pointers to FFIVector instead of bool to
	store Polonius output data.
	* checks/errors/borrowck/polonius/rust-polonius.h (FFIVector__new):
	Helper function.
	(FFIVector__new_vec_pair): Likewise.
	(FFIVector__new_vec_triple): Likewise.
	(FFIVector__push): Likewise.
	(FFIVector__push_vec_pair): Likewise.
	(FFIVector__push_vec_triple): Likewise.
	* checks/errors/borrowck/rust-borrow-checker.cc (BorrowChecker::go):
	Convert FFIVector to std::vector representation for easier
	navigation.
	* checks/errors/borrowck/polonius/rust-polonius.cc: New file,
	implementation of helper functions.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>