Implement line-number reporting for failing THEN1/>-

This relies on a somewhat disgusting quotation filter hack which
replaces occurrences of >-/THEN1 in line with the following pattern:

tac1 >- tac2 - - - > tac1 >>- linenumber ?? tac2

This relies on >>- and ?? being left-associative, which is how they
are defined. The definition of ?? is

fun f ?? x = f x

The linenumber is inserted by the quotation filter, which tracks this
naturally.

Fix interactive unquote to handle Control-C better

In particular, it's possible to interrupt a quotation parse, and not
then be stuck in the state of quotation-parsing for the rest of the
session.

Make quotation handling more efficient

In particular, dependency analysis on files does not need to read the
entirety of the file into memory first.

Make Moscow ML dep. calculation unquote-free as well

This allows yet more pleasant refactoring (with the
FNameToUnquotedDeps module completely disappearing). Now there is a
generally useful QFRead module in tools/Holmake that allows the
quotation machinery to be applied to strings and files.

The quotation filter is still a "push" technology: it consumes
everything it can and outputs as it goes. (It does block if its input
stream blocks of course.) I think it would be nice to modify the
filter (now QuoteFilter in tools/Holmake) so that it was "pull", the
client getting to pull chunks of output a bit at a time.

Currently, the API supports "reader" operations of type unit -> char
option, and these look like pull operations, but these are pulls on a
buffer that has already been filled with as much output as possible.

Quote filter now outputs via user-provided fns

Previously it targeted a TextIO.outstream, only assuming output and
flush operations.

Use polyc for all compilation in configure.sml

I don't think I can use polyc in the creation of a new heap that is done
within buildheap though, which means that I believe we still have to
tell users to build poly with --enable-shared.

Progress with #292

Minor tidying in quotation filter code.

Tweaks following release of Poly/ML 5.5.1.

Modify poly configuration so that it builds the quotation filter out
of the tools/quote-filter directory. This allows deletion of the copy
of quote-filter in tools-poly.

Can now build a quotation filter (called in this directory
quote-filter; later called unquote, or unquote.exe in the current
build process) in PolyML.