arkworks feature requires u64_backend

[redshiftzero]

Currently in the decaf377 repository the following should work:

• No default features, using the u32_backend (for hardware wallets)
• Default features, using the u32_backend,r1cs features (for WASM)

However, when r1cs is enabled, arkworks is enabled, which incorrectly forces the use of the u64_backend: https://github.com/penumbra-zone/decaf377/blob/main/src/fields/fp.rs#L16

Discovered in: penumbra-zone/web#1492

[redshiftzero]

The default 64-bit field arithmetic backend requires the "optional" feature arkworks, which is why the build without default features (i.e. without arkworks) in #108 are currently failing CI

One path forward is to make the 32-bit backend become the default (i.e. adding a u64_backend feature and bumping major release versions) since it doesn't have the arkworks feature requirement