Introduce rust_unop_ind_operation

Introduce rust_unop_ind_operation

This adds class rust_unop_ind_operation, which implements UNOP_IND for
Rust.  Rust requires a special case here to handle trait objects.

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

	* rust-lang.c (eval_op_rust_ind): No longer static.  Add "opcode"
	parameter.
	(rust_evaluate_subexp): Update.
	* rust-exp.h (class rust_unop_ind_operation): New.