(!! new API lib) Use a new generated PEG parser

In addition to being written clearly in PEG, the new parser supports sequences-of-expressions (that is, the new semicolon-delimited sub-expressions.)

This requires a huge refactoring of the entire codebase (upcoming), but will lead to a much cleaner and clearer reactor implementation.

Squashes: 041a5c6..54322d4, 7e092b8, 5c13078
(- tests re) Seriously improving delegated() tests
(- fix) delegated now properly handles built-ins
(new API inc) Integrate the new parser with the API

There's quite a bit more work left to do to integrate this new API with everything. This is only the start.

Squashes: aa0e336..dfb9612, 2a2fdc0..07f3be1, 55bbc10a, 42efdf4
(new tests) Exercise some of the new parser-integration

Squashes: 12b10c3, 3b252fa
(- fix API) Re-implement the shebang stripping

This introduces `Paws.parse.prepare()`, replacing the `root: true` option to the old parser.
(- new API) Sequence#at() dives into wrapped expressions
(!! re API) Completely re-implement Execution and advance()

`Execution` is now built to utilize the new script forms from the new parser.

Most notably, advance() is wayyy more clear than it previously was (although I'm still a far cry from *happy* with it.)

Squashes: 869ba0f, a1b2dd4, 6a3f785..e859eaf, 73a35e5..370438c, 7c70009
(- tests fix) Re-write tests to depend on the new Execution

Squashes: ffd2cbd, 08da4cc
(new tests) Partially exercise the new #advance()

Squashes: ed62d72, 53b9a4d
(- fix) Fix Execution#complete() for the new script-types
(re fix completes:5d03d) reactor: Handle locals-null signifier

This wraps up the last of the obviously-broken fallout from the parser refactor. :D
(fix CLI) interact: Re-write to use the new parser
(- fix CLI) Repair executable's --expression flag