18d8b0e469 Replace VarInt type with ReadExt and WriteExt functions (Steven Roose)
003db025c1 Return encoded length from WriteExt::emit_slice (Steven Roose)
Pull request description:
This the meat and potatoes out of Steven's work in #2133 and also closes#1016
ACKs for top commit:
apoelstra:
ACK 18d8b0e469 successfully ran local tests
Tree-SHA512: 2df96c91e0fbfdc87158bde9bbdd9565f67e3f66601697d0e22341416c0cd45dd69d09637393993f350354a44031bead99fd0d2f006b4fc6e7613aedc4b0a832