Guarantee trivial copy constructor and destructor for Str and Slice

[dtolnay]

This enables the System V ABI to pass them in registers. See page 20 of https://software.intel.com/sites/default/files/article/402129/mpx-linux64-abi.pdf.

If a C++ object has either a non-trivial copy constructor or a non-trivial destructor¹¹, it is passed by invisible reference (the object is replaced in the parameter list by a pointer that has class POINTER)¹².

An object with either a non-trivial copy constructor or a non-trivial destructor cannot be passed by value because such objects must have well defined addresses. Similar issues apply when returning an object from a function.

Repro:

const char *ptr(rust::Str str) { return str.repr.ptr; }
size_t len(rust::Str str) { return str.repr.len; }

Before:

ptr(rust::Str):
        mov     rax, qword ptr [rdi]
        ret
len(rust::Str):
        mov     rax, qword ptr [rdi + 8]
        ret

After:

ptr(rust::Str):
        mov     rax, rdi
        ret
len(rust::Str):
        mov     rax, rsi
        ret

This change is also observable by std::is_trivially_copy_constructible<rust::Str>() (https://en.cppreference.com/w/cpp/types/is_copy_constructible) which lets us hit better fastpaths in libraries downstream.