Refactor LLVM Orc JIT to actually enable the lazy compilation and speedup
the launching process:
https://llvm.org/docs/ORCv2.html#laziness
Main modifications:
- Create LLVM module for each wasm function, wrap it with thread safe module
so that the modules can be compiled parallelly
- Lookup function from aot module instance's func_ptrs but not directly call the
function to decouple the module relationship
- Compile the function when it is first called and hasn't been compiled
- Create threads to pre-compile the WASM functions parallelly when loading
- Set Lazy JIT as default, update document and build/test scripts
