Research TypeScript and migration methods

[MirandaWood]

Using a statically typed language is standard for compilers, and would improve our class management!
We should do this before making large changes to the logic as it requires touching every bit of js code - will (hopefully) be easier to add features in ts than add them in js and upgrade later.

This will be a big job so will assign as an epic.

[MirandaWood]

Solc-js (compiler wrapper* for Solidity -> abi, ast, etc.) migrated/is migrating to ts as well. Their process will probably be useful to check out: ethereum/solc-js#287

Plus, Consensys have an AST traverser written in ts: https://github.com/ConsenSys/solc-typed-ast
This will likely help us change the NodePath and other classes to ts