Compiling serde v1.0.208
error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv7em-none-eabi` target may not support the standard library
  = note: `std` is required by `serde` because it does not declare `#![no_std]`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`