Implement falliable and infallible conversion from String

[lkdm]

Implement fallible and infallible conversion from String.

Infallible (implement core::str::FromStr):

1. Strip out alphabetic and special characters, except - and ..
2. Strip out all . characters after the first.
3. Perform banker's/half-even rounding up to third decimal place.
4. Cast as given Int type
5. Return in Mill Newtype.

Fallible

1. Return a conversion error if it's not able to be parsed.

Important: Do not depend on std library.