Consider to rewrite ILasm/ILdasm in C#

During the cleanup of PAL simulated I/O functions, ILasm/ILdasm are the remaining components that using them heavily. The codebase of these tools are already quite a mess that written in ancient style (C++ 9x). It appears that a complete rewrite can provide more benefit and clean up more utility code.

The benefits for using native code instead of managed code:
- Better support for source-build bootstrapping
- Use the same loading/emit code of runtime (DNMD in the future)

The plan is to rewrite one first (probably ildasm) and test round-tripping with counterpart of old version.

@jkotas @jkoritzinsky @AaronRobinsonMSFT Any thoughts? Do you think it's better to use native code or managed code?