parser: Add parsing of auto traits

This adds enough handling to start parsing `auto` traits but not handle
them in the AST, lowering phase or HIR yet.

The feature is named `optin_builtin_traits` in Rust 1.49 but changes to
`auto_traits` later down the line. So we'll need to take care of this later
on.

Finally, this also changes the way the lexer detects if a string is a
keyword or not. We relied on a call to `std::lower_bound` to figure
out if a string was contained in an array or not, and this ended up
causing issues when adding new keywords. We can instead switch to a
simple hashmap and search for the key. The code *might* be less
optimized (unsure) but it is definitely simpler and easier to read.

Fixes #1814

gcc/rust/ChangeLog:

	* ast/rust-item.h (class Trait): Add `has_auto` field.
	* checks/errors/rust-feature.cc: Add handling for `feature(optin_builtin_traits)`
	* checks/errors/rust-feature.h: Likewise.
	* lex/rust-lex.cc: Fix keyword classification using hashmap.
	* lex/rust-token.h: Add `auto` keyword token.
	* parse/rust-parse-impl.h (Parser::parse_vis_item): Parse auto traits
	on `auto` keyword.

gcc/testsuite/ChangeLog:

	* rust/compile/auto_trait_invalid.rs: New test.
	* rust/compile/auto_trait_valid.rs: New test.

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