Solidifying core API design direction

- updating model for base ClrMdPack script pack context class so that it
represents a single active session under control
- shortened method names to make API succint (e.g.
AttachToProcess->Attach, DetatchFromCurrentProcess->Detatch)
- added ability to write to an IOutputWriter (current implementation is
only ConsoleOutputWriter)
- added support in the IScriptPack::Terminate to detach the current
ClrMdPack session if it is attached (exit REPL without calling Detatch?
no problem.)
- started adding set of DumpXXX methods which, in addition to the raw
APIs that will be available, actually leverage those APIs and dump
meaningful info to the IOutputWriter (which today is just
ConsoleOutputWriter for use with REPL)