Python bindings: provide context managers for the Blocks

Expose EDSC block builders as Python context managers, similarly to loop
builders.  Note that blocks, unlike loops, are addressable and may need to be
"declared" without necessarily filling their bodies with instructions.  This is
the case, for example, when branching to a new block from the existing block.
Therefore, creating the block context manager immediately creates the block
(unless the manager captures an existing block) by creating and destroying the
block builder.  With this approach, one can either fill in the block and refer
to it later leveraging Python's dynamic variable lookup

    with BlockContext([indexType]) as b:
      op(...)  # operation inside the block
      ret()
    op(...)  # operation outside the block (in the function entry block)
    br(b, [...])    # branching to the block created above

or declare the block contexts upfront and enter them on demand

    bb1 = BlockContext()  # empty block created in the surrounding function
    bb2 = BlockContext()  # context
    cond_br(bb1.handle, [], bb2.handle, [])  # branch to blocks from here
    with bb1:
      op(...)  # operation inside the first block
    with bb2:
      op(...)  # operation inside the second block
    with bb1:
      op(...)  # append operation to the first block

Additionally, one can create multiple throw-away contexts that append to the
same block

    with BlockContext() as b:
      op(...)  # operation inside the block
    with BlockContext(appendTo(b)):
      op(...)  # new context appends to the block

which has a potential of being extended to control the insertion point of the
block at a finer level of granularity.

PiperOrigin-RevId: 238005298