srfi-140 (immutable strings) - getting close to last call

[Per Bothner <per at bothner dot com>]

The specification has been updated:
https://srfi.schemers.org/srfi-140/srfi-140.html
The most important changes:
- A redone and greatly expanded Implementation section.
- Link to a test suite.
- Renamed string-replicate to xsubstring, to match SRFI-13.
- Added unintentionally-skipped string-pad{,right} and string-trim{,-right,-both}.

The Kawa implementation is complete (barring bugs and testing) and seems
to be working well.  So far very little Scheme code is broken by the
switch to immutable strings, but more testing would be welcome.  When testing,
please replace (import (scheme base)) by (import (kawa base)) to make
sure you get the istring-returning procedures.

The implementation is in the 'invoke' branch of the Kawa repository:
https://gitlab.com/kashell/Kawa/tree/invoke
The plan is before finalization to have a final kawa 2.4 release, and then
merge the invoke branch into master.  (This will becomes kawa 3.0 in the near future.)

The documentation for the 'invoke' branch is here:
http://per.bothner.com/kawa/invoke/index.html
The updated strings chapter is here:
http://per.bothner.com/kawa/invoke/Strings.html

To do before finalization:
- Add some more tests, including testing for srfi-118 features,
  and more non-BMP-characters.
- Merge the Kawa invoke branch into master.
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/