This commit adds a pair of encoder/decoder to take the advantage of the
BigSize format when encoding an uint64 and possibly an uint32.
Often the time an uint64 value is not big enough to fill all the 8
bytes, thus using BigSize can save extra bytes when save it to db. And
for uint32, if we know most of the values do not exceed 65536, we can
also save at least 1 byte using BigSize format.
This commit introduces `MakeBigSizeRecord` that can be used optionally
where db space is a concern.
Tlv is used more widely in lnd than just for the onion payload. This
commit isolated the protocol-specific odd/even logic, so that tlv can be
used freely elsewhere. An example of this use is db serialization.
This commit adds an additional return value to
Stream.DecodeWithParsedTypes, which returns the set of types that were
encountered during decoding. The set will contain all known types that
were decoded, as well as unknown odd types that were ignored.
The rationale for the return value (rather than an internal member) is
so that the stream remains stateless.
This return value can be used by callers during decoding to make
assertions as to whether specific types were included in the stream.
This is need, for example, when parsing onion payloads where certain
fields must be included/omitted depending on the hop type.
The original Decode method would incur the additional performance hit of
needing to track the parsed types, so we can selectively enable this
functionality when a decoder requires it by using a helper which
conditionally tracks the parsed types.
