Add non-local scoping

[BobVarioa]

Currently:

• Adds lexical scoping! Only works with variables. this and new.target are not supported in this PR.
• Fixes var and this semantics  #107
• Fixes [meta] Implement globalThis #108
• Update expressions work for more things now, i.e this.x++
• const is now actually different from let
• No more var/let a = () => ... hack (except for test262), const a = () => ... is still optimized however
• Also most scope related behavior is disabled in precompile, mainly for compilation time reasons.

Caveats:

• Breaks delete for variables and builtin vars, which already had limited support
• Slows down compile time by a bit, mainly because of lifting up names (we have to traverse the ast twice).

Also, this is a somewhat broken implementation, but for good reasons in my opinion. For example, take this:

function test() { 
  let a = 1; 
  return () => a++;
} 
let b = test(); 
b();
test();
b(); // should return 2, returns 1 currently

And well, that's not how that is supposed to work at all, but adding that requires runtime allocation, and binding certain values to functions. I will follow up this PR implementing them both, but I felt that they were somewhat out of scope of this PR.

To do:

• Fix some Test262 issues (+0.54 🎉)
• Make sure precompile works
• Throw reference errors earlier instead of later
• Lift vars out of nested scopes

[BobVarioa]

Still not quite ready to merge, but changes from here will be relatively minor.