Note that `detail` was replced with `function_signature` to avoid
calling `from` on FunctionSignature twice.
I didn't add new tests because the current ones seem enough.
This introduces the new type -- Semantics.
Semantics maps SyntaxNodes to various semantic info, such as type,
name resolution or macro expansions.
To do so, Semantics maintains a HashMap which maps every node it saw
to the file from which the node originated. This is enough to get all
the necessary hir bits just from syntax.
