Entrypoint string specification and validation

Right now the entrypoint strings are '#'-delimited (Wasm file and method): https://github.com/containerd/runwasi/pull/102. In https://github.com/containerd/runwasi/pull/219 there was some discussion about specifying this particular entrypoint string format and semantics well, so it would be documented for users and other projects could potentially support this.

In particular, we need to write validation code and tests. For example, #219 listed these test cases:

1. `foo#bar`
2. `#bar`
3. `foo#bar#bar`
4. `foo`
5. `#foo#bar`
6. `foo#`
7. `foo#bar; -- %`