Increase the initial size of StringInfo buffers to 1024 bytes (from 256);

Increase the initial size of StringInfo buffers to 1024 bytes (from 256);
likewise increase the initial size of the scanner's literal buffer to 1024
(from 128).  Instrumentation of the regression tests suggests that this
saves a useful amount of repalloc() traffic --- the number of calls occurring
during one set of tests drops from about 6900 to about 3900.  The old sizes
were chosen in the late 90's with an eye to machines much smaller than
are common today.