Remove num-traits dependency on std

[dlkj]

   Compiling num-traits v0.2.14
error[E0463]: can't find crate for `std`
  --> /home/dkj/.cargo/registry/src/github.com-1ecc6299db9ec823/num-traits-0.2.14/src/lib.rs:21:1
   |
21 | extern crate std;
   | ^^^^^^^^^^^^^^^^^ can't find crate
   |
   = note: the `thumbv6m-none-eabi` target may not support the standard library