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This improves performance on the test.yaml attached to #158 by about
25% on my machine as compared to the previous commit (0.25s -> 0.20s),
as measured by `time build/util/parse < test.yaml > /dev/null`.

Switched the 'pragma once' to only happen on MSVC, or gcc >= 3.4 (it was causing trouble on the sun compiler)

Flipped the include guard and the pragma, and don't use the pragma for early versions of gcc (< 3.4)

Fixed flow folding, and made the separation slightly cleaner (but the whole scanscalar thing could use a major refactoring)

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.