31ba1af74a Remove unused (and broken) functionality in SpanReader (Pieter Wuille)
Pull request description:
This removes the ability to set an offset in the `SpanReader::SpanReader` constructor, as the current code is broken since #23653. All call sites use `pos=0`, so it is actually unused. If future call sites need it, `SpanReader{a, b, c, d}` is equivalent to `SpanReader{a, b, c.subspan(d)}`.
It also removes the ability to deserialize from `SpanReader` directly from the constructor. This too is unused, and can be more idiomatically simulated using `(SpanReader{a, b, c} >> x >> y >> z)` instead of `SpanReader{a, b, c, x, y, z}`.
This was pointed out by achow101 in https://github.com/bitcoin/bitcoin/pull/23653#discussion_r763370432.
ACKs for top commit:
jb55:
crACK 31ba1af74a
achow101:
ACK 31ba1af74a
Tree-SHA512: 700ebcd74147628488c39168dbf3a00f8ed41709a26711695f4bf036250a9b115574923bbf96040ec7b7fee4132d6dbbcb5c6e5a2977c4beb521dc1500e6ed53
b7e63306e8