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Instead of deriving different tokens from a base Token class, we now use an enumerated TOKEN_TYPE to distinguish types. This is so we don't have to cast all the time when parsing the resulting token stream.
Also, removed start/end stream tokens.

Renamed the valid/possible tokens to a single variable status with enums valid, invalid, and unverified.

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There's a bug (and question): should we test simple keys' validity BEFORE stuff or AFTER stuff?

Also messed around with multiple character peeking on the input, and got something working.

We're now using exceptions for errors, and scanning/pushing tokens is exception-safe (using a set of "limbo tokens").