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.
Note: we meed to skip the trivia filter to make sure that
`covers!(call_info_bad_offset)` succeeds otherwise we exit call_info
too early.
Also the test doesn't pass: `FnCallNode::with_node` always detects
a MacroCall.