mutable_borrow_reservation_conflict future compat warning

[bjorn3]

When compiling wasmer using rustc 1.36.0-nightly (a784a8022 2019-05-09) there is a mutable_borrow_reservation_conflict future compatibility warning emitted:

warning: cannot borrow `state.fs.fd_map` as mutable because it is also borrowed as immutable
   --> lib/wasi/src/syscalls/mod.rs:792:5
    |
790 |     let fd_entry = wasi_try!(state.fs.fd_map.get(&from).ok_or(__WASI_EBADF));
    |                              --------------- immutable borrow occurs here
791 | 
792 |     state.fs.fd_map.insert(
    |     ^^^^^^^^^^^^^^^ mutable borrow occurs here
...
796 |             rights: fd_entry.rights_inheriting,
    |                     -------------------------- immutable borrow later used here
    |
    = note: #[warn(mutable_borrow_reservation_conflict)] on by default
    = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
    = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>