refactor utils

[aarondill]

Refactors the tabnine.utils module to add types and fix some minor issues.

The list of commits is below. Each commit should include a description of the change (or be self-evident).

• fix: remove global in select_range
• fix: allow nil arguments to debounce function
• refactor: add some basic types
• fix: allow nil values in subset()
• refactor!: remove utils.script_path and replace with utils.module_dir
• refactor!: current_position now returns two values
• refactor!: ends_with and starts_with types. Remove special case for str==''
• refactor: finish off types

[amirbilu]

Thanks for the contribution @aarondill, this is a big change which needs to be heavily tested.. might worth to split to multiple PRS?

[aarondill]

@amirbilu sorry for never responding.

I can split this up (and probably should). I can see this being split into three PRs:

1. script_dir -> module_dir
2. allow nil in argument list in debounce
3. types

the first two would be fairly small, but the third would still have a large (green) diff, as non of the functions currently have types, and would now all need comments describing the types.

[aarondill]

I'm closing this PR. The behavioral changes have been done in #164, #165, and #166

I'll reimplement the rest of the types after those have been merged (to avoid conflicts)