mimalloc-lite

This version is simplified from Daan Leijen and the Microsoft Research team's mimalloc.

• Linux-only support (removed macOS, Windows, Apple, WASI)
• Assumes GCC/Clang (removed MSVC and other compiler branches)
• Removed MI_PADDING, MI_SECURE, MI_ENCODE_FREELIST, and MI_DEBUG>2
• Renamed obsoleted malloc family functions (e.g. pvalloc, cfree)
• Removed additional interceptors like strdup and realpath. Their libc implementations typically call the interceptable malloc.
• Removed C++ new/delete interceptors
• Removed mi_register_deferred_free