Fixes and change to `lst` constructor

build_and_install.sh now works, no longer uses outdated “spring” path.

Empty functions (like `{}`) now no longer cause the interpreter to crash

`get` and `sub` now work properly (the index mechanism was broken
before)

`let` now expects an iterator, which it uses to populate the newly
created lst.  Use `lst []` for an empty list.
Added boolean operators

`true & x` returns `x
`false & x` returns `false`

`true | x` returns `true`
`false | x` returns `x`
Added length and exit methods

Length added to strings

Exit added to top-level environment