Make the Rust parser pure

Make the Rust parser pure

This makes the Rust parser a pure parser and removes all the
(non-constant) globals from rust-exp.y.  This seemed like a nice
simplification to me and I think it should probably be applied to all
the parsers.  Perhaps it would be good to go even one step farther and
have all parsers derive from parser_state.

Tested on x86-64 Fedora 26.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* rust-exp.y: Now a pure parser.  Update all rules.
	(%union): Move earlier.
	(current_parser, work_obstack): Remove globals.
	(rust_parser, ~rust_parser): Update.
	(class rust_parser) <copy_name, concat3, crate_name, super_name,
	lex_character, lex_number, lex_string, lex_identifier,
	rust_lookup_type, convert_params_to_types, convert_ast_to_type,
	convert_name, convert_params_to_expression,
	convert_ast_to_expression, ast_basic_type, ast_operation,
	ast_compound_assignment, rust_op, ast_literal, ast_dliteral,
	ast_structop, ast_structop_anonymous, ast_unary, ast_cast,
	ast_call_ish, ast_path, ast_string, ast_struct, ast_range,
	ast_array_type, ast_slice_type, ast_reference_type,
	ast_pointer_type, ast_function_type, ast_tuple_type>: New methods.
	(rust_parse): Update.
	(rustyyerror, rustyylex): Add parser parameter.
	(rust_lex_test_one, rust_lex_int_test, rust_lex_exception_test)
	(rust_lex_stringish_test, rust_lex_test_sequence)
	(rust_lex_test_trailing_dot, rust_lex_test_completion)
	(rust_lex_test_push_back, rust_lex_tests): Update.