parse: Add parse_file function ...

By using this, the Parser class is temporary and all parsing state is
contained according to the call stack without any possibility of
external mutation.

parse: Put parsing state and functions in its own class

parse: Move parser state down the file

parse: Instead of parser_skip just use parser_next

parse: Handle end of file better ...

Instead of throwing an error, treat end of files as a specific type of
token that's added to the end of the tokenizer.
This also means we can always return EOF once we reach the end of the
tokens just by sticking to the last token.

parse: Make verbs optional in statements ...

When a statement contains only a subject, return the subject.
This is pointless for bare statements, but very useful for set
statements.

parse: Fix 'Expect to' to be 'Expected To'

parse: Make symbols values case sensitive

parse: Represent values as a literal or reference ...

Instead of just passing back the token, instead decide what the value is
now and let the interpreter do something with it later.

Right now it just does the same thing it did before, but hopefully it
will turn text literals in to references to objects.

parse: Replace 'tokenizer' with 'tokenize'

parse: Make tokens case-sensitive ...

This should reduce ambiguity, especially since they read differently to
both eyes and screen readers.

main: Move parsing code to parse.py