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