Implement Rust field operations

Implement Rust field operations

This implements the field operations STRUCTOP_STRUCT and
STRUCTOP_ANONYMOUS, for Rust.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (eval_op_rust_struct_anon, eval_op_rust_structop):
	No longer static.
	* rust-exp.h (class rust_struct_anon): New.
	(class rust_structop): New.