This commit adds concrete encoding methods for primitive integral types.
When external libs need to create custom encoders, this allows them to
do so without incurring an extra allocation on the heap. Previously, the
need to pass a pointer to the integer using an interface{} would cause
the argument to escape, which we avoid by having them copied directly.
This varint has the same serialization as the varint in btcd and
bitcoind, but has different behavior wrt returned errors. In order to
ensure the inner loop properly detects cleanly written records,
ReadVarInt will not only return EOF if it can't read the first byte, as
that means the reader has zero bytes left.
It also modifies the API to allow the caller to provided a static byte
array, which can be reused across all encoding and decoding and
increases performance.
