Rename shift operators.

Adding "ByScalar" to the current set of shift operators makes room in
the namespace for adding vector-vector shifts in the future.
Update and reorganize README.md
Add a brief disclaimer.
int32x4 vector-vector shifts
int16x8 and int8x16 vector-vector shifts
Mention vector-vector shifts in the README
Signed and unsigned conversion to int32x4
Fix copy+pastos in comments
Conversions to and from unsigned int32x4