StringOfSpecifiedLengthUnparser renamed.

StringOfSpecifiedLengthUnparser renamed.

Code modified to reflect that this only truncates.

New name is StringMaybeTruncatedUnparser.

Fix center padding bug, remove unneeded & buggy padding code from prim.

Fix round trips that use utf-8, lengthUnits characters, padding and nil.

Remove debug variable

Remove ThinThrowable from UnparseError.

These are fatal errors, so we might as well get a decent backtrace
from them.

Removed problematic withBitLengthLimit on data output streams.

This concept is just broken for unparsing.

Enabled round trip on test.

Incorporated feedback on Review 662

DFDL-1600, DFDL-1325, DFDL-1599, DFDL-1579, DFDL-1603, DFDL-1557