Use a variable-length integer for many serialization wrappers

Use a variable-length integer for many serialization wrappers

The lightning protocol uses u16s for lengths in most cases. As our
serialization framework primarily targets that, we must as well.
However, because we may serialize objects that have  more than 65K
entries, we want to be able to store larger values. Thus, we define
a variable length integer here which is backwards-compatible but
treats 0xffff as "read eight more bytes".

This doesn't address any specific known issue, but feels like good
practice just in case.