Fix mixed-size memcpy_s

[xeren]

Modifies the intrinsics for the string.h standard library functions to be compatible with mixed-size accesses. With the current default settings, dynamic memory accesses are torn into units of archType. E.g. memcpy(dest, src, 16) translates to

bv64 __memcpy_1 = load(bv64 dest)
store(bv64 src, bv64 __memcpy_1)
bv64 __memcpy_2 = load(bv64 dest + (bv64) 8)
store(bv64 src + (bv64) 8, bv64 __memcpy_2)

With the option --mixedSize=true, more informed tearing is applied later. So that call translates to

bv128 __memcpy#1 = load(bv64 dest)
store(bv64 src, bv128 __memcpy#1)

WIth this PR, that scheme also applies to memcpy_s, memcmp and memset. This should fix at least #977.

[hernanponcedeleon]

I do not like this kind of code. If we expect the result register to be of type integer (otherwise we will get some exception somewhere else when trying to create the zero value with null type), we should have a proper precondition to the method

[ThomasHaas]

I think the code looks a lot better now, especially with the clarification what a Slice is.
I have not checked all details, but I think it can be merged.

[hernanponcedeleon]

errorCodeSuccess -> having error/success together is weird. I would call it returnCode