Elide 'foo: foo' in struct literals where possible.

Elide 'foo: foo' in struct literals where possible.

Another thing I only just realised: although in general you're
supposed to explicitly say the name of each field you're initialising,
there's a special case where you can leave it out if the name of the
local variable you're initialising it _to_ is exactly the same.
Presumably on the grounds that that still doesn't allow much
confusion, and is terser.