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This follows existing pattern for object constructor API and allows to
create memoryview objects e.g. in external modules.

Might be useful at least for memoryview hacks.

This helps make files reusable across other ports.

See issue #1122.

Just to reduce code size.  Messages are still to the point and
unambiguous.

So that navite emitter passes (comprehensions use yield which is not yet
supported by native emitter).

Should only give an error on the last pass of the assembler, since
that's when we are certain about the branch size.

Addresses issue #1143.

Addresses issue #1140.