Rename crate table-engine to mito

[evenyag]

Now the mito table engine is implemented in the table-engine crate. I think it would be much clear if we rename this crate to mito, since table-engine sometimes is easy to confuse with the table crate, which defines our API for a table engine.

Using table-engine is much like a historical reason as we hadn't decided the name for our default table engine at the time we started to write the engine. Another benefit is mito only takes one word and table-engine takes two.

[MichaelScofield]

Aye!

[waynexia]

How about mito-engine? Only mito may look unclear that what is inside.

[evenyag]

Well, there are two options:

• mito
• mito-engine

[evenyag]

I think mito is enough, as innodb, WiredTiger.

[evenyag]

Closed by #539