_d_aaGetY

fnV * _d_aaGetY(K, V, T : V1[K1], K1, V1, K2)(auto ref scope T aa, auto ref K2 key, out bool found)

Lookup key in aa. Called only from implementation of (aa[key]) expressions when value is mutable.

`aa`	associative array
`key`	reference to the key value
`found`	returns whether the key was found or a new entry was added

if key was in the aa, a mutable pointer to the existing value.

If key was not in the aa, a mutable pointer to newly inserted value which is set to zero