Update ENVIRONMENT

[shawwn]

environment is now a table rather than a list of tables. The parent
frame of the current environment is (get environment '%parent).

When implementing an interpreter for Lumen, I noticed that the current
design makes it costly to store lexical environments. Each closure
would need to store (cut environment), i.e. a clone of the
environment array.

Performance is almost identical: the new design is less than 1% slower
on LuaJIT, Lua, and Node.

I'm not sure this PR is worth merging. I dislike that the name
"%parent" is now reserved. If someone tries to define a macro or
special form named "%parent", it will break.

The good part is find-environment and current-environment. These
functions hide the details of Lumen's environment frames.

I'll submit this for now and think it over.

An idea to investigate: a get-properties function which takes an
arbitrary object or function and returns a table. That'd make it easy
to annotate environment frames, e.g. marking a frame as "the toplevel
of the current function", or "the module toplevel," etc). It'd be
similar to elisp's symbol-plist, but for objects/functions rather
than symbols.