Let `Radix2Domain::offset` to be `FpVar` instead of `F`

[tsunrise]

Description

Current Radix2Domain does not support non-constant offset. This PR adds that functionality.

Highlighted Changes:

• Radix2Domain will be renamed to Radix2DomainVar
• Radix2DomainVar will offset that is FpVar instead of F
• Radix2DomainVar::interpolate_and_evaluate will specialize according to whether self.offset is constant or not.

closes: #64

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• [x] Wrote unit tests
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• [x] Added a relevant changelog entry to the Pending section in CHANGELOG.md
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[weikengchen]

The change looks good. Although this is a breaking change, the Radix2Domain in r1cs_std is not used very often downstream (most are using the Radix2Domain in ark-ff), and I think renaming makes sense.

[weikengchen]

will wait for @ValarDragon for a final review