FastSerializer: Simplify OUString write a bit more

Change-Id: Ifa0746d635ec43cdc37867cf94bcc128bb8e2aca
inline TokenMap::getUtf8TokenName

Change-Id: Icd9c6ebc9feb3e7aba28b01729b582a8f49c832a
FastSerializer: Also use cache for writing to ForMerge if we are inside mark()

To ensure the correct order of calling ForMerge methods,
call flush always before touching maMarkStack.

This was the missing piece in optimizing write() methods,
because of writeBytes() checking each time what to call.
E.g. for Calc documents we don't use maMarkStack at all.
So, just transfer the output to proper "ForMerge" when inside mark()
and allow optimizations.
This commit makes write() methods almost 1/3 as fast.

Change-Id: I96c13888206c81f87e29b998839f78ea9d5570af
FastSerializer: Add simpler writeByte method for single characters.

Change-Id: If1637e9e72270c0dfe8afea1362cad24e7a3f1cd