Partition Lean binaries into reasonably-sized shared libraries

[Kha]

We currently link lean and leanpkg statically (as much as possible), which makes for very simple distribution. Interestingly, we can still dlsym symbols from these executables for use in the interpreter, but this does come with some caveats on specific platforms. Additionally, we are regularly hitting the maximum number of symbols exportable from a Windows DLL/PE (2^16). If we want to keep all (relevant) symbols reachable from the interpreter, we should partition the Lean & C++ code into reasonably-sized shared libraries, which will also reduce code duplication between lean and leanpkg (and potential future executables).

[leodemoura]

Let's do this one after we refactor the compiler and delete some of the old C++ code.