Add simpler macro versions of ULE/VarULE macros to reduce `syn` deps

[Manishearth]

See #2310

In https://github.com/unicode-org/icu4x/issues/5124 we're trying to slim down ICU4X's deps. A lot of our crates don't need the full power of the ule/varule macros, and it would be nice to expose MBE macro versions of them.