Add general `bls_signed_wrapper` function

[vbuterin]

Status quo:

class Foo(Container):
    bar: Hash
    baz: ValidatorIndex
    qux: BLSPubkey

class SignedFoo(Container):
    message: Foo
    signature: BLSSignature

Proposal:

class Foo(Container):
    bar: Hash
    baz: ValidatorIndex
    qux: BLSPubkey

SignedFoo = bls_signed_wrapper(Foo)