aa-init.dd

Added `initialize` function for associative arrays

This function allocates memory for an empty associative array (AA). It
is useful to avoid copying a null AA reference. Then
when AA elements are added through the original reference, the copied
reference will still point to the same AA:
---
int[int] aa;
auto b = aa;
assert(b is null);
aa.initialize;
assert(aa !is null);
auto c = aa;

aa[4] = 4;
assert(b != aa);
assert(c is aa);
---