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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.

Moved the three scalar token scanning functions back to scantoken.cpp, so scanscalar.cpp now only has the main scalar scanning function.
Renamed ScanScalarInfo to ScanScalarParams.

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

Moved the input stream, together with line/column info, into its own class, which allowed some other stuff just to pass the stream, and not have to be a member of Scanner.

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

Refactored some common whitespace-parsing code in scanning both scalars.
Implemented the flow collection tokens.

Split off the specific regular expressions, and the specialized token-scanning functions, into their own files.