While we should handle completion for isolated file, it's better
achieved by using empty Analysis, rather than working only with &File:
we need memoization for type inference even inside a single file.
Previously, module was synonym with a file, and so a module could have
had several parents. This commit introduces a separate module concept,
such that each module has only one parent, but a single file can
correspond to different modules.
Implements a pretty barebones function signature help mechanism in
the language server.
Users can use `Analysis::resolve_callback()` to get basic information
about a call site.
Fixes#102
