[mlir][python] Add generic operation parse APIs

Currently the bindings only allow for parsing IR with a top-level
`builtin.module` op, since the parse APIs insert an implicit module op.
This change adds `Operation.parse`, which returns whatever top-level op
is actually in the source.

To simplify parsing of specific operations, `OpView.parse` is also
added, which handles the error checking for `OpView` subclasses.

Reviewed By: ftynse, stellaraccident

Differential Revision: https://reviews.llvm.org/D143352