glsl_symbol_table: Add new talloc-based new()

glsl_symbol_table: Add new talloc-based new()

We take advantage of overloading of the new operator (with an
additional parameter!) to make this look as "C++ like" as possible.

This closes 507 memory leaks when compiling glsl-orangebook-ch06-bump.frag
when measured with:

	valgrind ./glsl glsl-orangebook-ch06-bump.frag

as seen here:

	total heap usage: 55,623 allocs, 14,389 frees
	(was 13,882 frees before)