This is part of a larger effort to improve the memory layout of AST
nodes of the self-hosted parser to reduce wasted memory. Reduction of
wasted memory also translates to improved performance because of fewer
memory allocations, and fewer cache misses.

Compared to master, when running `zig fmt` on the std lib:

 * cache-misses: 801,829 => 768,624
 * instructions: 3,234,877,167 => 3,232,075,022
 * peak memory: 81480 KB => 75964 KB