refactor: don't reexport specl.std functions from specl.util.

* lib/spec/util.lua (M.champ, M.escape_pattern, M.merge)
(M.Object, M.prettytostring, M.process_files, M.slurp)
(M.tostring, M.totable): Remove.
* lib/specl/matchers.lua, lib/specl/shell.lua,
specs/spec_helper.lua: Adjust. Require specl/std.lua into a
local table, and access functions directly.
* specs/custom_formatters_spec.yaml (context with an accumulator):
Set local tostring directly from specl/std.lua.

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