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.

Saves around 60 bytes code on Thumb2 archs.

Saves 116 bytes for stmhal and 56 bytes for cc3200 port.

Reduces code size by 72 bytes on Thumb2 archs.

This is rarely used feature which takes enough code to implement, so is
controlled by MICROPY_PY_ARRAY_SLICE_ASSIGN config setting, default off.
But otherwise it may be useful, as allows to update arbitrary-sized data
buffers in-place.

Slice is yet to implement, and actually, slice assignment implemented in
such a way that RHS of assignment should be array of the exact same item
typecode as LHS. CPython has it more relaxed, where RHS can be any sequence
of compatible types (e.g. it's possible to assign list of int's to a
bytearray slice).

Overall, when all "slice write" features are implemented, it may cost ~1KB
of code.

Saves about 250 code bytes for Thumb2 archs.

This is so all memory requests go through the same interface.

This makes exception traceback info self contained (ie doesn't rely on
list object, which was a bit of a hack), reduces code size, and reduces
RAM footprint of exception by eliminating the list object.

Addresses part of issue #1126.

Slightly smaller code, and does not need to use C stack to save
temporaries.