minor patch submission: CREATE CAST ... AS EXPLICIT

Hello,

Please find attached a minor stylish patch. It compiles and the update
test cases work for me.

Description:

Add "AS EXPLICIT" to "CREATE CAST"

This gives a name to the default case of "CREATE CAST", which creates a
cast which must be explicitely invoked.

From a language definition perspective, it is helpful to have a name for
every case instead of an implicit fallback, without any word to describe
it. See for instance "CREATE USER CREATEDB/NOCREATEDB" or "CREATE RULE ...
DO ALSO/INSTEAD" for similar occurences of naming default cases.

--
Fabien.