matchers: new contain.a_permutation_of adaptor.

* specs/matchers_spec.yaml: Specify expected behaviour of a new
adaptor.
* lib/specl/matchers.lua (Object, prettytostring, totable):
Alias directly from specl.std, to save reexporting functions
through specl.util all the time.
(clone, empty, size): Alias these too.
(concat): Use " and " as the final element separator when
concatenating a_permutation_of arguments.
(matchers.container.format_alternatives): Format strings
correctly when called by a_permutation_of.
(matchers.container:a_permutation_of?): New adaptor.
* docs/specl.md (Unordered matching with a_permutation_of):
Document it.
* TODO.yaml: Some new TODO items.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: preload sources, and prefer preloaded from env.require.

On my machine this gets me about a 5% speedup in a large set
of specfiles, but I have an SSD which makes opening sources
every time they are loaded seem fast - on an io-bound machine
this should show an even greater speedup.
* local.mk (bin/specl): When generating this file, preload the
library sources with verbatim function initialisers.
* lib/specl.lua (env.require): Prefer preloaded modules, rather
than a full blown manual file slurp and loadstring().
And, excepting "spec_helper" and temporary lua_xxxxxx files, add
the result of loadstring to package.preload to speed up subsequent
loads.

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