Implement fast decimal integer encoding.

Compared to 'blaze-textual' and the Integer 'Show' instance the 'integerDec'
builder is 2x faster for small integers that fit into a machine word, and it
is 4x faster for large integers. This performance improvement speeds-up the
encoding performance of libraries like 'aeson' that use 'Integer's to
represent numbers.
Finish renaming 'Bounded/FixedEncoding ~> Bounded/FixedPrim'.

  FE ~> FP
  BE ~> BP
  fixedEncoding   ~> fixedPrim
  boundedEncoding ~> boundedPrim