tab-completion implemented for Kawa

[Per Bothner <per@bothner.com>]

I checked in an implementation of command-completion (tab-completion)
for Kawa.  This is context-senstive - for example the following works
when using the JLine2 input-ediing library:

#|Kawa:1|# ab<TAB>s
abs              absolute-path?
#|Kawa:1|# abs<ENTER>
#<procedure abs>

Kawa wills in the 's' and gives you two options; <ENTER> choose "abs".

What makes it interesting is that it is content-dependent,
and includes bindings in the lexical scope:

#|Kawa:2|# (let ((absol 12)) abso<TAB>l
absol            absolute-path?

It works by running the compiler in tentative mode
with "(let ((absol 12)) abso<COMPLETE>" as the input.
Here <COMPLETE> is a special private-use Unicode character
that tells the Kawa name-lookup framework to look for names
starting with "abso".

This "tentative mode" could also be useful for on-the-fly
error checking, syntax coloring, use/definition cross-references.
But that is for another day.

Currently this only works with the JLine2 library.
You have to:
    ./configure --with-jline2=/path/to/jline2.jar

However, most of the logic is independent of JLine2, and
is in the file gnu/expr/CommandCompleter.java.  It should be
doable (for-example) to have an Emacs frontend call the
CommandCompleter#complete method.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/