Respect constant names in validation rules

[Serhii Potapov]

As for now it's not possible to use constant name in the most of validation rules.

Examples

use nutype::nutype;

const NAME_MIN_LEN: usize = 3;

#[nutype(
    sanitize(trim)
    validate(min_len = NAME_MIN_LEN)
)]
pub struct Name(String);

previous errors
error: Expected usize, got `NAME_MIN_LEN`
  --> src/main.rs:12:24
   |
12 |     validate(min_len = NAME_MIN_LEN)

Or

const AMOUNT_MAX: f64 = 42.0;

#[nutype(
    validate(max = 42.0)
)]
pub struct Amount(f64);

error: Expected f64, got `AMOUNT_MAX`
  --> src/main.rs:11:20
   |
11 |     validate(max = AMOUNT_MAX)

Reasons

The original reason for this was to be able to validate correctness of the given values at compile time.

For example, if min is greater than max, the following example does not compile:

#[nutype(
    validate(max = 10, min = 20)
)]
pub struct Number(i32)

error: `min` cannot be greater than `max`.
       Sometimes we all need a little break.
 --> src/main.rs:9:14
  |
9 |     validate(max = 10, min = 20)
  |              ^^^

Desired

• Allow specifying the values for the validation rules as named constants
• Keep the validation, if it can be executed at compile time
• Consider generating extra tests to ensure the the dynamic (named constant) values still make sense at runtime.
• Use this trick to ensure that max is greater that min: TedDriggs/darling#244 (comment)