fix all AA operations in wasm

[brianush1]

In wasm, a struct that wraps a single value is treated differently than just that single value by itself in the calling convention. This was causing issues when functions such as _aaLen would be called with an Impl* value, which was then trying to be re-interpreted as an AA, meaning that the function would get garbage input.

This PR removes the struct AA definition, since it was just a wrapper around a single value, and thus fixes these issues.

[skoppe]

Hi thanks for submitting this.

Interesting... I was actually just looking at those pesky AA's myself.

I do wonder if this shouldn't be fixed language-wide. There are likely more places where people use single field structs. In any case, let me try it out over the next few days.

[skoppe]

I had a look at this. It indeed seems that in some cases the calling convention doesn't add up. I did a quick tests with some standalone functions and a little struct with a pointer to replicate the invalid calling convention, but could not. It seems to be a problem specific to the AA's, likely because of separate compilation and extern(C).

I worked around it by replacing all AA aa parameters with Impl* impl in aaA.d. Then I added a little AA aa = AA(impl); at the top of each function.

I intend to fix it by diving deeper into the calling convention mismatch, but I first want to finish passing all (applicable) druntime and phobos tests first, as well as updating to the latest version of druntime/phobos/ldc.

When I have the code pass the tests I will push here.