Add a list of symbols for stable standard library crates

There are a few locations where the crate name is checked against an
enumerated list of `std`, `core`, `alloc`, and `proc_macro`, or some
subset thereof. In most of these cases, all four crates should likely be
treated the same. Change this so the crates are listed in one place, and
that list is used wherever a list of `std` crates is needed.

`test` could be considered relevant in some of these cases, but
generally treating it separate from the others seems preferable while it
is unstable.

There are also a few places that Clippy will be able to use this.

compiler/rustc_span/src/lib.rs

@@ -67,7 +67,7 @@ mod span_encoding;
 pub use span_encoding::{DUMMY_SP, Span};
 
 pub mod symbol;
-pub use symbol::{Ident, MacroRulesNormalizedIdent, Symbol, kw, sym};
+pub use symbol::{Ident, MacroRulesNormalizedIdent, STDLIB_STABLE_CRATES, Symbol, kw, sym};
 
 mod analyze_source_file;
 pub mod fatal_error;