More refined cursor context analysis

An expression is now better defined in terms of allowed tokens. This
fixes the issues with parsing things like `x := fmt.#`, which is
currently broken (ref #212).
Fix typo: editted -> edited.
Log using the "log" package when in debug mode.
Log extracted expression tokens.
Run `go fmt` on `_gccgo/package.go`.
Rename test input files.

Append ".in" suffix to syntactically invalid test input files. This
makes my local triggers happy (otherwise they will try to `go fmt` them
and fail, and it's becoming annoying now).
add "autobuild" option (no-op for now)
autobuild skeleton