Rather than using bytes.NewBuffer, which is a read/write entity
(io.ReadWriter), use bytes.NewReader which is only a read entitiy
(io.Reader) in all cases where it is possible. Benchmarking shows it's
slightly faster and it's also technically more accurate since it ensures
the data is read-only.
There are a few cases where bytes.NewBuffer must still be used since a
buffer with a known length is required for those instances.
This commit changes the InvVect_* constants, which are not standard Go
style, to the InvType*. In order to preserve backwards compatibility, it
also adds a legacy.go file which maps the old public constant names to the
new ones.
Closes#1.
