Move encoding files to its own module/folder

[ValarDragon]

The existing IAVL code is very poorly structured, getting more files out of the root directory will only help

We should move encoding and encoding_test to a single encoding folder. We would have to make every function in encoding.go exported which is fine.

This has no external API breaks, as all these functions are internal only.

[tac0turtle]

this should be moved to internal/encoding. This makes it public to the repo and private when used by others. Easier to fix The API this way.

[tac0turtle]

cosmos#460 here is the pr for this.