These encoders can be composed to create composite types without
incurring additional allocations that would be required to pass the
truncated types through the generic interface.
This commit fixes a bug in DTUint16 and DTUint32, which would cause them
to read too many bytes from the reader. This is due to the fact that
ReadFull was being called on a slice that could be greater than the
underlying type. This is not an issue for DTUint64, since the 8-byte
buffer corresponds to the maximum possible size of a uint64. The
solution is to clamp the buffer to 2 and 4 bytes respectively.
A series of tests are also added to exercise these cases.
