fix(p3): remove body accessors

[rvolosatovs]

Currently, there are two ways to acquire a handle to the incoming request/response body handle:

1. Call body method on the request/response
2. Call into-parts static function on the request/response

Assume pseudocode:

body := response.body()
stream := body.stream()
stream.read(buf)
drop(stream)
drop(body) // Forgetting this would cause a trap
(_, body) := Response::into_parts(response)

There a few issues in the example above.

1. It's very easy to make a mistake and cause a runtime trap, developers must very closely follow the WIT definition and read documentation to avoid a crash at runtime.
2. The host must do a lot of work and state tracking to e.g. ensure underlying body stream state synchronisation. If body method can be called at most once, that simplifes the task and potentially, the host could move the ownership of the body into the returned resource and move it back on drop of the body resource. Regardless of the approach and semantics, introducing multiple accessors for the body requires increased complexity in the host.

To fix this, simply remove the body accessors. This way only a single way to acquire a body resource exists, which eliminates the possibility of misusing the interface by the application developer and simplifies implementation in the host.

This is an alternative solution to #159, will mark that PR as draft.
Closes #159

[lukewagner]

Depending on the resolution to #156, that might justify the continued existence of the body accessor.