malloc-bind: Make Malloc a trait, use in elfmalloc, add documentation

[joshlf]

• Make Malloc a trait rather than a struct so that method implementations can be overridden at the trait level, allowing the macros to still be useful even with custom implementations
• Implement Malloc for ElfMallocGlobal
• Switch elfc to use malloc-bind
• Add documentation to each Malloc method to document the exact behavior of the corresponding function of the C API; this should not only provide documentation for callers, but also provide documentation for those intending to provide custom implementations of any of these methods
• Remove reallocarray, which was spuriously compiled for Linux (it is actually currently an OpenBSD-only function)
• Fix more bugs in implementations
• Change macros to use absolute paths for identifiers to address this issue: https://users.rust-lang.org/t/how-to-import-in-a-macro-without-conflicting/12785
• Use the correct minimum alignment on Windows and Mac

[ezrosent]

Did some performance smoke tests on the later commits. LGTM