refactor: Use serialize functions in getInitialWitness

[LHerskind]

Fixes #2625. Using the functions defined in serialize when applicable.

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[sirasistant]

This is great! One less place to remember to update when we change inputs

[sirasistant]

Maybe we can add a type guard for isACVMField that checks that it's a hex value of the correct length and if is ACVMField just short circuit the conversion to/from buffer? It'd remove any performance impact of this refactor :D

[LHerskind]

The size and such should already be handled by the fromString that was why I just ended up using that 👀. The frombuffer is reading 32 bytes, so if it is of wrong size should break there.

  /**
   * Create a Fr instance from a hex-encoded string.
   * The input 'address' can be either prefixed with '0x' or not, and should have exactly 64 hex characters.
   * Throws an error if the input length is invalid or the address value is out of range.
   *
   * @param address - The hex-encoded string representing the field element.
   * @returns A Fr instance.
   */
  static fromString(address: string) {
    return Fr.fromBuffer(Buffer.from(address.replace(/^0x/i, ''), 'hex'));
  }

[sirasistant]

Oh yeah I wasn't meaning as validation, It's because, suppose that the toACVMField receives an ACVMField (where it could just return the parameter without any operation)
Then we now would be doing:
Fr.fromString() => internally does Fr.fromBuffer() and Buffer.from(string)
Then fr.toBuffer()
Then buffer.toString('hex')

so that's many conversions to/from buffer when if it's sure about the type it can just return the parameter without doing anything. So with a type guard, you could:

function isACVMField(value: any): value is ACVMField {
  ... // Check here if hex string and a hex regex with the ACVMField length for example
}

// Then in toACVMField
...
if (isACVMField(value)) {
    return value
}
... The rest of the function

But maybe it's not that efficient if we need to check in runtime if it's an ACVMField with a regex ): Maybe I'm overcomplicating myself. The truly zero runtime cost would be to convert ACVMField to a branded string but that's more painful