maint: move specl and specl.lua to a new src directory.

Previously, when setting the path to .../specl/?.lua so that
`require "specl"` works, it also means that `require "std"` will
find `.../specl/std.lua` instead of lua-stdlib's installed version.
* specl/specl.in, specl/specl.lua: Move from here...
* src/specl.in, src/specl.lua: ...to here.
* specl/specl.mk: Move all but nobase_dist_lua_DATA from here...
* src/src.mk: New file. ...to here.
* specl/.gitignore: Move /specl from here...
* src/.gitignore: New file. ...to here.
* Makefile.am: Include new src.mk makeflie fragment.
* specs/formatters_spec.yaml, specs/matcher_spec.yaml,
specs/specl_spec.yaml: Adjust for new location of specl program.
* rockspecs.lua: Make sure we install to the directory LuaRocks
knows about.