This change makes use of the new code generator rlp/rlpgen to improve the
performance of RLP encoding for Header and StateAccount. It also speeds up
encoding of ReceiptForStorage using the new rlp.EncoderBuffer API.

The change is much less transparent than I wanted it to be, because Header and
StateAccount now have an EncodeRLP method defined with pointer receiver. It
used to be possible to encode non-pointer values of these types, but the new
method prevents that and attempting to encode unadressable values (even if
part of another value) will return an error. The error can be surprising and may
pop up in places that previously didn't expect any errors.

To make things work, I also needed to update all code paths (mostly in unit tests)
that lead to encoding of non-pointer values, and pass a pointer instead.

Benchmark results:

    name                             old time/op    new time/op    delta
    EncodeRLP/legacy-header-8           328ns ± 0%     237ns ± 1%   -27.63%  (p=0.000 n=8+8)
    EncodeRLP/london-header-8           353ns ± 0%     247ns ± 1%   -30.06%  (p=0.000 n=8+8)
    EncodeRLP/receipt-for-storage-8     237ns ± 0%     123ns ± 0%   -47.86%  (p=0.000 n=8+7)
    EncodeRLP/receipt-full-8            297ns ± 0%     301ns ± 1%    +1.39%  (p=0.000 n=8+8)

    name                             old speed      new speed      delta
    EncodeRLP/legacy-header-8        1.66GB/s ± 0%  2.29GB/s ± 1%   +38.19%  (p=0.000 n=8+8)
    EncodeRLP/london-header-8        1.55GB/s ± 0%  2.22GB/s ± 1%   +42.99%  (p=0.000 n=8+8)
    EncodeRLP/receipt-for-storage-8  38.0MB/s ± 0%  64.8MB/s ± 0%   +70.48%  (p=0.000 n=8+7)
    EncodeRLP/receipt-full-8          910MB/s ± 0%   897MB/s ± 1%    -1.37%  (p=0.000 n=8+8)

    name                             old alloc/op   new alloc/op   delta
    EncodeRLP/legacy-header-8           0.00B          0.00B           ~     (all equal)
    EncodeRLP/london-header-8           0.00B          0.00B           ~     (all equal)
    EncodeRLP/receipt-for-storage-8     64.0B ± 0%      0.0B       -100.00%  (p=0.000 n=8+8)
    EncodeRLP/receipt-full-8             320B ± 0%      320B ± 0%      ~     (all equal)