Improve code quality and performance

[Dandigit]

Issues as small as passing by value instead of reference to major design flaws in classes plague the code in this repository, and must be addressed.

This is the reference implementation of Enact, so first and foremost, it must work. But as a piece of code, it should also be correct, idiomatic and readable. More commenting needs to be done throughout the entire codebase.

Plus, it needs to run fast, without leaking memory. There's a long road ahead in terms of optimization that needs to be done.

It's certainly no easy feat, but I'm sure that over time, these issues can be gradually addressed and resolved.

[Dandigit]

#65 addresses some issues regarding the usage of shared_ptr as opposed to unique_ptr in the AST. Also gets rid of the really hacky string reading code that the analyser used to look up typenames.

[Dandigit]

#70 removes all global static classes, including Enact and GC, instead using a new Context class which manages all aspects of the interpreter.