Implement dictionary

[quephird]

The following changes were made in this PR for initial support of dictionaries in Lox:

• Dictionary literals in Lox are like the ones in Swift, where commas delimit pairs and the colon delimits each key and value, e.g. ["a": 1, "b": 2]
• Because any LoxValue can be a dictionary key, it needed to be Hashable since LoxDictionary uses Swift dictionary as a backend for managing key/value pairs
• That meant that I needed a sensible strategy for uniquely identifying all cases of LoxValue including those associated with UserDefinedFunctions. And so I introduced an objectId property with a UUID type that accomplishes the task
• I needed to add explicit implementation for == for Expression and ResolvedExpression because lists of tuples are not automatically Equatable. Equatable conformance for expressions and statements, and their resolved counterparts, is necessary only for unit tests
• Moved standard library into its own file to reduce the amount of noise in Interpreter
• Lox dictionaries have a native properties, count, keys, and values, as well as native methods removeValue() and merge()
• Like lists, dictionaries also support a subscript syntax for retrieving and values associated with keys
• There is a utility method, Interpreter.makeDictionary() that is used both in the context of the interpreter as well as for unit tests