Koda-validate

**Optimization**
- Use `pattern.match(...)` instead of `re.match(pattern, ...)` in `RegexPredicate` and `EmailPredicate`

**Features**
- `ValidationResult.map()` can be used to succinctly convert data contained within `Valid` objects to some other type or value

**Breaking Changes**
- `to_serializable_errs` produces different text, removing assumption that validation input was deserialized from json

**Improvements**
- Allow `to_serializable_errs` to accept callable for next level

**Bug Fixes**
- [30](https://github.com/keithasaurus/koda-validate/issues/30) validate_signature ignores validators' coercion and processors

**Maintenance**
- General updates to dev dependencies

**Bug Fixes**
- `UnionValidator.typed`'s `overloads` specify that arguments should be positional-only

**Docs**
- extend docs for `UnionValidator` and `NTupleValidator`

**Features**
- Runtime type checking via `koda_validate.signature.validate_signature`
- Coercion customizable via `Coercer` (and helper decorator `coercer`)

**Maintenance**
- Minor bug fixes in examples/