mesa: Add iterate method for string_to_uint_map

mesa: Add iterate method for string_to_uint_map

The upcoming shader cache needs this to be able to cache hash data
from the gl_shader_program structure.

Edited-by: Carl Worth <cworth@cworth.org>:
There is an internal implementation detail that the hash table
underlying the struct string_to_uint_map stores each value internally
as (value+1). The user needn't be very concerned with this (other than
knowing that a value of UINT_MAX cannot be stored) since put() adds 1
and get() subtracts 1.

So in this commit, rather than call the user's function directly with
hash_table_call_foreach, we call through a wrapper that fixes up the
off-by-one values before the caller's callback sees them.

And with this wrapper in place, we also give a better signature to the
callback function being passed to iterate(), so that this callback
function can actually expect a char* and an unsigned argument, (rather
than a couple of void* ).

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>