At some stage we named the compact encoding `VarInt` (which makes sense
because the compact size encoding is a variable length integer encoding).
However it turns out the term "varint" is used in Core for a different
encoding so this may lead to confusion.
While we fix this naming thing observe also that the `VarInt` type is
unnecessarily complicated, all we need to be able to do is encode and
decode integers in compact form as specified by Core. We can do this
simply by extending our `WriteExt` and `ReadExt` traits.
Add `emit_compact_size` and `read_compact_size` to emit and read compact
endcodings respectively.
Includes addition of `internals::compact_size::encoded_size_const`.
Patch originally written by Steven, Tobin cherry-picked and did a bunch
of impovements after the varint vs compact_size thing (#1016).
ref: https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer
Co-developed-by: Tobin C. Harding <me@tobin.cc>
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