Update MLtonVector.adoc

Update unfoldi to be correct and make sense from a type perspective
Update MLtonArray.adoc

Update unfoldi to be correct and make sense from a type perspective
Merge pull request #72 from robsimmons/refactor-arrays

Refactor array and vector implementation into a primitive implementation (using SeqInt.int for indexing) and a wrapper implementation (using the default Int.int for indexing).
Merge pull request #73 from robsimmons/unfoldi

Give MLton.{Vector,Array}.unfoldi a correct description in the guide.
Merge branch 'master' of github.com:mlton/mlton into intinf-alt