MS3

The highlights of this iteration were fleshing out the memory to handle
composite values (arrays and strings, called AddressableValues in the code),
and refactoring PlangGrammar to be more robust.

I'm happy with how the memory turned out: arrays are little addressable pieces
of memories themselves, making the whole thing a nice treelike structure. I'm
less happy with the PlangGrammar changes: while it's better than it was, it
really starts feeling overengineered. Also, while a separate typechecker object
was expected to be created in this sprint, it wasn't: it will start to really
matter only when function resolution comes in.

Features

Lots of types: strings, characters, booleans, floats, and also fixed-size
arrays, including multidimensional arrays. Conditional statements.

Design

• Better memory model, composite values.
• Conditional statements.
• Lexical elements of PLanG now come from a property file, making language
  variants possible.

Refactor

• Exploded PlangGrammar: the recursive descent parser is implemented through a
  collection of parser classes with a single Parse static function. Not very
  pretty design, but definitely nicer than the huge PlangGrammar.java was.
• AST nodes keep their location information.
• The names of PlangGrammar's symbols are now in an enum.
• Things (types, variables) are registered in Context by a Name object
  (instead of by plain string keys).