In https://youtu.be/qvIZZf5dmTE, we've noticed that AstIdMap does a
linear lookup when going from SyntaxNode to Id. This leads to
accidentally quadratic overall performance. Replace linear lookup with a
O(1) hashmap lookup.
Future work: don't duplicate `SyntaxNodePtr` in `AstIdMap` and switch to
"call site dependency injection" style storage (eg, store a
`HashSet<ErasedFileAstId>`).
See the explanation of the work here on YouTube
https://youtu.be/wvEgymUm7cY :-)
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