Initial interpreter and standard library essentials

[alexrp]

In the interest of being able to run Celerity code ASAP and getting a suite of behavior tests done, the initial interpreter implementation will lean heavily on the .NET runtime for garbage collection and data structures (BigInteger, List<T>, Dictionary<TKey, TValue>, HashSet<T> etc). Eventually, these components will be swapped with native ones shared between the interpreter and JIT compiler.

Some essentials of the standard library will need to be implemented - mostly just stuff for manipulating the various data types of the language and interacting with agents.

Partially depends on:

• Design and implement the shared linear IRs (HIR, MIR, LIR) #60