Implement Pad/Fill (Trimming of padding)

Perform entity replacement on Pad/Fill String Literals. This was saved until later as we are not using Pad/Fill at the moment.

DFDL-13-042R: Text String Pad Character
-020R: textTrimKind
-041R: textStringJustification
-046R: textNumberJustification
-047R: textNumberPadCharacter
-131R: textBooleanJustification
-132R: textBooleanPadCharacter
-165R: textCalendarJustification
-166R: textCalendarPadCharacter

Also: DFDL-7-089R (padding is trimmed before escape scheme is applied.)

Also: This came from an errata to the spec:

If dfdl:nilKind is ‘literalValue’ then nilValue specifies a white space
separated list of DFDL literal strings that are the possible
representation values for nil. On parsing the element value is nil if
the trimmed data matches one of the literal strings in the list.