str: preallocate "0" .. "9" strings
str: preallocate "0" .. "9" strings

Since these are relatively common strings we can preallocate them and return
them instead of going to the heap.

One of the consumers of libjeffpc saw over 2% reduction in struct str
allocations during startup:

cache                        buf     buf     buf     buf  memory     alloc alloc
name                        size  in use  in ptc   total  in use   succeed  fail
------------------------- ------ ------- ------- ------- ------- --------- -----
str-cache (before)            24    5342       -    5577    132K    628246     0
str-cache (after)             24    5342       -    5577    132K    614861     0

Note: This consumer is a heavy user of strings.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc