package_ext: don't perturb the global environment by default.

For backwards compatibility, `require "std"` will still write
symbols into the global namespace, but when loaded directly be
much more hygienic and return everything in a table, like most
other modules:
* src/package_ext.lua (M): Define module symbols in this table
and return it.
* src/std.lua.in: Inject package_ext module symbols into global
namespace.
* specs/package_ext_spec.yaml: Specify new behaviour, being
careful about `require "std"` side-effects.
Merge pull request #23 from rrthomas/gary/ext-hygiene

package_ext: don't perturb the global environment by default.