Fix unaligned dereferencing (preparation for Rust upgrade)

[luc-blaeser]

Analysis:

• Unaligned pointer dereferencing has undefined behavior in Rust: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
• Using 64-bit fields in a struct implies that the struct is also 64-bit-aligned. This is however not guaranteed in our heap as objects are only 32-bit aligned.
• The recent Rust version occasionally checks misaligned pointer dereferencing. This currently fails in Stream.rs and in BitmapIterator.rs for the Compacting GC. However, it is correctly aligned in the incremental GC.
• Simplification: The implicit padding for 64-bit field alignment depending on the header size and the GC (incremental or not) is now removed.

Intended as a fix for the build errors in #3947.

[luc-blaeser]

Update: I also resolved an issue with the wasm-profiler that showed an error due to an unsupported WASM instruction for number conversion.

[github-actions]

Comparing from cf341ee to 759db5f:
In terms of gas, no changes are observed in 4 tests.
In terms of size, 4 tests regressed and the mean change is +0.0%.

[crusso]

LGTM

We'll need to be careful that @matthewhammer's regions PR doesn't introduce more 64-bit object fields.

[luc-blaeser]

Comparing from 66c55a1 to f2aaa0b: In terms of gas, 2 tests regressed, 2 tests improved and the mean change is +0.0%. In terms of size, 4 tests improved and the mean change is -12.4%.

The performance of this PR without Rust compiler upgrade has been remeasured in #3999. No difference compared to master.

[ggreif]

I like how you divide the 64-bit members into two consecutive 32-bit ones, and how that avoids padding. Thanks for doing this!

[luc-blaeser]

I like how you divide the 64-bit members into two consecutive 32-bit ones, and how that avoids padding. Thanks for doing this!

Thank you also very much for reviewing!

[luc-blaeser]

LGTM

We'll need to be careful that @matthewhammer's regions PR doesn't introduce more 64-bit object fields.

Thank you very much for the review. Yes, we need to be careful about future inserts of 64-bit fields. Once we have moved to new Rust version, the error would probably be reported at runtime.