Allowing the for member syntax to enumerate key/value pairs in a map.

Given the new `for member` syntax support for enumerating over the items in
an array, it would be useful to have an easy way of enumerating over the
key/value pairs in a map. This could be done through the same `for member`
syntax, but the members for a map would be a 2-item sequence or a map of
the key/value pairs in the map in an implementation-defined stable order.

If the variant of this proposal to use a map for the key/value pairs is
used, then given a map of type `map(K, V)` the member RecordTest would be
`record(key as K, value as V)`. For a `map(*)` type, the member RecordTest
would be `record(key, value)`.

Combined with sequence/map/array decomposition, this would allow a user to
write expressions like:

    for member ${key, value} in $map
    return ``[`{$key}`=`{value}`]``

It may be useful to have a different keyword to denote maps, such as
`entry` like JavaScript and Java, or `item` like Python, but using `member`
for both arrays and maps would be more consistent with the user wanting to
enumerate over the content of those data structures.

Kind regards,
Reece