Fix undefined behaviour in View::tryCopyTo overlap

[tobil4sk]

This was causing random failures in the haxe test suite: #1286 (comment)

The part where it was going wrong was here: https://github.com/HaxeFoundation/haxe/blob/3aed86e98aaa6097c3b8c9ab5c1aee5f70faa2f9/tests/unit/src/unit/TestBytes.hx#L47

The new implementation uses memcpy for this blit, which is undefined behaviour since the source and destination regions overlap. See: https://en.cppreference.com/w/cpp/string/byte/memcpy.html

If any of the following conditions is satisfied, the behavior is undefined:
[...]
- Copying takes place between objects that overlap.

std::memmove is the alternative which is safe to use for overlapping blocks: https://en.cppreference.com/w/cpp/string/byte/memmove.html. This is also what is used in the ArrayBase::Blit method:

hxcpp/src/Array.cpp

Lines 179 to 182 in 4cff17a

	if (src+len < dest || dest+len<src)
	memcpy(dest,src,len);
	else
	memmove(dest,src,len);

[Aidan63]

Ah, nice catch! I forgot that memcpy wasn't safe in overlaps and wasn't thinking about that case at all.

[tobil4sk]

Looks like ci got reran but just in case it comes up again, we had this failure during execution of the mac arm64 "haxe" test:

Memory exhausted.
/Users/runner/work/_temp/d85ca6f0-43ca-4142-8d9d-60a39b2041fe.sh: line 1:  5788 Trace/BPT trap: 5       bin/TestMain
 try HXCPP_GC_BIG_BLOCKS.