Add tail calls to the interpreter

[tekknolagi]

This means we can evaluate bigger factorials (for example) without
blowing the Python call stack.

[tekknolagi]

This PR becomes much clearer if you view files and click the gear and click "hide whitespace"

[gregorybchris]

This PR becomes much clearer if you view files and click the gear and click "hide whitespace"

This is an unreal pro tip

[tekknolagi]

This PR becomes much clearer if you view files and click the gear and click "hide whitespace"

This is an unreal pro tip

Also, git diff -w

[tekknolagi]

I want to work on the API for this a little more. It feels fragile.

[tekknolagi]

Going to try and bring this back from the dead. Seems quite useful and I think this is a fine API