Compilation error with no-std + serde + validation

[Indy2222]

This macro #[nutype(validate(less_or_equal = 100), derive(Deserialize))] causes a compilation error when used in no_std environment. The error is due to the use of format!() macro (which depends on alloc) when reporting an error during serde de-serialization. See the minimal example in this repository: https://github.com/Indy2222/nutype-nostd-serde

Compilation error:

error: cannot find macro `format` in this scope
 --> src/lib.rs:4:1
  |
4 | #[nutype(validate(less_or_equal = 100), derive(Deserialize))]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the attribute macro `nutype` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `nutype-issue` (lib) due to 1 previous error

I think this should be fixed, as both Nutype and Serde support no_std environments.

[greyblake]

@Indy2222 Thank you for the report!