MutableString: fix speelong mistake.

lib/zile/MutableString.lua: s/An (MutableString)/A \1/

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
FileString: new name for EStr, now with LDocs.

* bin/zmacs, bin/zz: Remove `require "zile.estr"`.
Adjust all callers.
* lib/zile/estr.lua: Moved from here...
* lib/zile/FileString.lua: ...to here.
Adjust all callers.
* lib/zile/zile.mk (dist_pkgdata_DATA): Adjust.
* lib/zile/FileString.lua: Move object methods into shared
metatable to reduce object size, and speed up cloning.
Add LDocs.
* build_aux/config.ld (file): Add lib/zile/FileString.lua.
* local.mk (dist_classes_DATA): Add doc/zile.FileString.html.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: use a sentinel file for multi-target rules.

Be nice to parallel make.
* local.mk ($(srcdir)/doc): Factor out of...
($(dist_doc_DATA) $(dist_classes_DATA) $(dist_modules_DATA):
...here.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>