[Format] Initial arrow-internal specification of compute IR

This JIRA tracks the initial specification of Arrow's internal compute IR.

The discussion has taken place in the open, mostly in this google doc: https://docs.google.com/document/d/1C_XVOG7iFkl6cgWWMyzUoIjfKt-X2UxqagPJrla0bAE/edit#heading=h.ie0ne0gm762l.