For a long time GHC has had some internal mechanism designed to support
a call-site inline directive, thus
	inline f xs
makes f be inlined at the call site even if f is big.

However, the surface syntax seems to have gone, and in any case it
can be done more neatly using a RULE.

This commit:
  * Removes the InlineCall constructor for Note
    and InlinePlease for SimplCont

  * Adds a new known-key Id called 'inline', whose definition in
    GHC.Base is just the identity function

  * Adds a built-in RULE in PrelRules that rewrites (inline f) to
    the body of f, if possible

  * Adds documentation

NOTE: I have not tested this (aeroplane work).  Give it a try!