Cast jsonb to numeric, int, float, bool

Now the simplest way to extract booleans and numbers from json/jsonb is
to cast it
to text and then cast to the appropriate type:

postgres=# select 'true'::jsonb::text::bool;
bool
------
t
postgres=# select '1.0'::jsonb::text::numeric;
numeric
---------
1.0

This patch implements direct casts from jsonb numeric (jbvNumeric) to
numeric, int4 and float8,
and from jsonb bool (jbvBool) to bool.

postgres=# select 'true'::jsonb::bool;
bool
------
t

postgres=# select '1.0'::jsonb::numeric;
numeric
---------
1.0

Waiting for your feedback.
If you find it useful, I can also add support of json and other types,
such as smallint and bigint.

--
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company