Refactor PlatformAgnostic interface into top-level module with no internal dependencies

Right now, there are two PlatformAgnostic folders -- one in lib\Common and one in lib\Runtime. These should be merged, ridded of Js:: namespace dependencies, and have a pluggable architecture similar to the Utf8Codex,

Sub-items of annoyance:

- [ ] lib\Common\PlatformAgnostic files aren't editable from within VS
- [ ] lib\Runtime\PlatformAgnostic files _shouldn't_ be used by ch and jshost, considering their internal dependencies, but in practice they are needed.
- [ ] Separating PlatformAgnostic out would make it much clearer that we can use the standard library in the implementation, since there is no risk of colliding with our garbage collector.