Patch: Allow substring/replace() to get/set bit values

Hi all,

attached a patch that adds the following functions for bit string:

- overlay
- get_bit
- set_bit

Some info:

1) overlay is implemented as calls to substring; given the different way substring behaves when used with strings vs bit strings:

test=# SELECT substring(B'1111000000000001' from 1 for -1);
substring
------------------
1111000000000001
(1 row)

test=# SELECT substring('1111000000000001' from 1 for -1);
ERROR: negative substring length not allowed

I don't think that this overlay implementation is what we want?

Example:

test=# SELECT overlay(B'1111' placing B'01' from 1 for 2);
overlay
---------
0111
(1 row)

(looks ok)

test=# SELECT overlay(B'1111' placing B'01' from 0 for 2);
overlay
-----------
111101111
(1 row)

????
This happens because substring(bit, pos, -1) means substring(bit, pos, length(bit string)),
and < -1 values for bit substring parameters are allowed: is this a bug in bit substring???

2) I tried implementing bit_get and bit_set as calls to overlay/substring:

DATA(insert OID = 3032 ( get_bit PGNSP PGUID 14 1 0 0 f f f t f i 2 0 23 "1560 23" _null_ _null_ _null_ _null_ "select (pg_catalog.substring($1, $2, 1))::int4" _null_ _null_ _null_ ));
DESCR("get bit");
DATA(insert OID = 3033 ( set_bit PGNSP PGUID 14 1 0 0 f f f t f i 3 0 1560 "1560 23 23" _null_ _null_ _null_ _null_ "select pg_catalog.overlay($1, $3::bit, $2, 1)" _null_ _null_ _null_ ));
DESCR("set bit");

but this doesn't give any check on the values provided:
that the bit looked for is in fact in the right range, and that the bit in set_bit is in fact a bit
(I don't like the idea of writing "select set_bit(B'01010111', B'1')" instead of
"select set_bit(B'01010111', 1)" ).
So I coded them in proper C internal functions.

Leonardo