Python bindings: introduce loop and loop nest contexts

Recently, EDSC introduced an eager mode for building IR in different contexts.
Introduce Python bindings support for loop and loop nest contexts of EDSC
builders.  The eager mode is built around the notion of ValueHandle, which is
convenience class for delayed initialization and operator overloads.  Expose
this class and overloads directly.  The model of insertion contexts maps
naturally to Python context manager mechanism, therefore new bindings are
defined bypassing the C APIs.  The bindings now provide three new context
manager classes: FunctionContext, LoopContext and LoopNestContext.  The last
two can be used with the `with`-construct in Python to create loop (nests) and
obtain handles to the loop induction variables seamlessly:

    with LoopContext(lhs, rhs, 1) as i:
      lhs + rhs + i
      with LoopContext(rhs, rhs + rhs, 2) as j:
        x = i + j

Any statement within the Python context will trigger immediate emission of the
corresponding IR constructs into the context owned by the nearest context
manager.

PiperOrigin-RevId: 237447732

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