Global variable initialization

This addresses #65 and adds support for tests like:

https://github.com/UCSD-PL/RefScript/blob/ssajoin/tests/pos/scope/init-04.ts
https://github.com/UCSD-PL/RefScript/blob/ssajoin/tests/pos/scope/init-05.ts

https://github.com/UCSD-PL/RefScript/blob/ssajoin/tests/neg/scope/init-05.ts

**New additions:**
- The notions of `Initialization`, which is now used in all SSA, TC and LQ, and refers to the state of WriteGlobal variables. There are two cases: `Initialized` and `Uninitialized` and the modifier propagates *locally* as a "must" fact:
```
var r; // global
if (...) { r = 1; }
```
Then `r` is conservatively considered uninitialized in the end.

- Variables that are still int he `Uninitialized` state the type used at USES is the "un-Kvared" type enhanced with a " + TUndef" -- a type system embedding of the fact that the variable "may" be undefined. So `envFinTy` gets two flavors, depending on whether we are writing to or reading an uninitialized variable.

- Every sight that initializes a global variable gets a new K-var. (I tried using the same K-var, but I was getting unsoundness -- which I was having trouble debugging). At join points the same trick with local SSAed variables is used. This means that we need to keep track of global-phi vars (this is done separately -but alongside- local var SSA tracking).

- Assigning to an `Uninitialized` global variable changes it's state to `Initialized`-duh-

- Ambient variables (those declared elsewhere and just imported) are considered Initialized. So a trick to avoid having uninitialized globs for which we don't want to give an initial value is to consider them as  ambient vars (by prefixing their declaration with `declared`).