chore: Add a feature gate to allow user disabling the NaN check

[dqhl76]

In the current implementation, there is an enforced check for NaN values during the serialization and deserialization of f32 and f64 because of cross platform problems. However, if user operate within homogenous environments where cross-platform compatibility is not a concern they cannot disable the check.

This PR add a feature gate to make this possible.

[dqhl76]

In the default setting, the NaN check will still be performed. The check is only skipped if the allow_nan feature is enabled.

[dj8yfo]

@dqhl76 we've thought about this a bit and decided to decline this PR.

One of the reasons is that a crate in cargo's dependency tree is resolved with a single set of features (not multiple sets of features). Thus using two crates in single project,
where one has borsh with this feature enabled and another has borsh with this feature disabled will result in single version of borsh with this feature enabled,
which may be incorrect behaviour for the second crate (which opens door to subtle hard-to-figure-out errors)

You may try using some wrapper types in client code like this, if you need to handle NaN-s:

use borsh::{BorshDeserialize, BorshSchema, BorshSerialize};
#[derive(Debug, BorshSerialize, BorshDeserialize, BorshSchema)]
enum F32Full {
    Value(f32),
    NaN,
}
impl From<f32> for F32Full {
    fn from(value: f32) -> Self {
        if value.is_nan() {
            Self::NaN
        } else {
            Self::Value(value)
        }
    }
}
fn main() {
    let source_vec: Vec<F32Full> = vec![f32::NAN.into(), 0.4342324f32.into()];
    let bytes = borsh::to_vec(&source_vec).unwrap();
    let output_vec: Vec<F32Full> = borsh::from_slice(&bytes).unwrap() ;
    println!("{:?}", output_vec);
}

[dqhl76]

@dj8yfo Thank you for your prompt review, and I understand your concerns regarding these changes. Close.