Added a GenotypeFieldValue copy of VariantFieldValue<int32_t>

First pass implementation of a `GenotypeFieldValue` with utility accessors like `field.is_hom_ref()`, `field.is_het()`, and `field.is_hom_var()` to avoid blocking other users.

Reviewers should watch out for syntax issues, for example when comparing to the `VariantFieldValue`, even after reading not 100% sure of

  * `(*this == other)` vs. `(this == &other)`
  * How `VariantFieldValue` is able to write to `const` values
  * etc.