precompilation support like RuntimeGeneratedFunctions.jl

So far caching GG's type encoding is unsafe when it comes to precompilation, as a result, several packages using GG need to consider some internal stuffs like implementing `__init__`.

GG v0.3 uses much less memory but makes this issue more severe.

The idea to address this is introducing per-package cache store(`like RuntimeGeneratedFunctions.init`) to work arbitrary precompilation. The way to integrate [GG v0.3's memory efficient caching](https://github.com/JuliaStaging/GeneralizedGenerated.jl/blob/master/src/ngg/compact.jl#L19) is making downstream packages hold their own cache pools and indices(shall not change after building the module), while those caches will be copied/merged into the big pool held by GG after downstream's module loading.