Switch from memmap to memmap2.

See https://rustsec.org/advisories/RUSTSEC-2020-0077

cranelift/filetests/src/function_runner.rs

@@ -9,7 +9,7 @@ use cranelift_codegen::{ir, settings, CodegenError, Context};
 use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
 use cranelift_native::builder as host_isa_builder;
 use log::trace;
-use memmap::{Mmap, MmapMut};
+use memmap2::{Mmap, MmapMut};
 use std::cmp::max;
 use std::collections::HashMap;
 use thiserror::Error;