Named tuple field support

[mratsim]

Nim serialization doesn't support named tuple fields.

For example we can't use:

  BLSSignMsg* = object
    input*: tuple[
      privkey: ValidatorPrivKey,
      message: seq[byte],
      domain: Domain
    ]
    output*: Signature

But have to introduce an extra object

  BLSSignMsgInput = object
    privkey*: ValidatorPrivKey
    message*: seq[byte]
    domain*: Domain

  BLSSignMsg* = object
    input*: BLSSignMsgInput
    output*: Signature

[zah]

I've tried to investigate this at one point, but the conclusion was that it cannot be supported at the moment. As tuples are passed along to functions, part of their type information is transformed to canonical form where the field names are no longer present.