Remove few pointless OUString::number() and one method

Change-Id: I3e9a302a7513eebfeff07402f71fc3dde22e4cc2
FastSerializer: Use -1 for unknown string length

Change-Id: I3920caf9d95f20992b7961873f1668468d797e8e
FastSerializer: Use fixed sized Sequence directly as cache

Well, at least the allocated space is fixed size.
When passing that to XOutputStream, change the size in a hacky way.

Change-Id: I24fa134286e3086beda25c9a6915549e7c69119a