refactor: use LuaMacro to support 'from table import <fieldlist>'.

* lib/larch/from.lua: New file. Implements 'from' syntax as a
LuaMacro macro.
* configure.ac (LUAM): Search for a working luam program.
* local.mk (LUAM_ENV): Add location of from.lua to LUA_PATH.
(LUAM_OPTS): Load from macro, and then dump macro-expanded
source.
(bin/specl): Expand macros in preloaded sources.
* lib/specl/formatter/progress.lua,
lib/specl/formatter/report.lua, lib/specl/formatter/tap.lua,
lib/specl/matchers.lua, lib/specl/shell.lua,
lib/specl/util.lua: Use 'from' to simplify alias creation.

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