rt: Remove last use of C++ exchange alloc
rt: Remove rust_exchange_alloc
rt: Remove indexed_list
rt: Remove timer
rt: Memory regions are never synchronized now
rt: Remove sync.h
rt: Remove exit_status helpers
rt: Move some test functions to rust_test_helpers
rt: Remove old precise GC code
rt: Remove rust_util.cpp
rt: Remove rust_abi
auto merge of #8705 : brson/rust/lesscxx, r=graydon