Decouple `VariantLookup` and variant-masking from `Primer3`

[msto]

It would be convenient if VariantLookup offered a method to mask arbitrary FASTAs and/or sequences based on the variants it contains.

Currently the logic to apply variant masking is contained within Primer3.get_design_sequences, which makes it difficult to reuse. I think this logic could be extracted and refactored into a method on VariantLookup.

[tfenne]

Sounds good to me.