Restore ppstream_funs as nullary type operator in term_pp_types

This name is used in user-written code (users' special purpose
pretty-printers), and there is no reason to rebind this name, forcing
them to change things.

Fix DecimalFractionPP

trailing newlines in *.{sml,sig} files from src/ removed

Trailing newlines from SML files in src/ were rendered in HTML documentation.

Move DecimalFractionPP code to num/theories so that it compiles with mosml.