Refactor sequences

Primitive sequences are indexed by SeqIndex.int, normal sequences
are indexed by Int.int. Introduces primitive sequences earlier on in the
dependency chain.

Changes also make the Span exception more primitive, exposes the unfold
function for arrays and vectors further, and reorder the sequence interface
so that sequence0.sig/sequence.sig and slice0.sig/slice.sig can be read as
parallel corpuses.
Merge branch 'master' into refactor-arrays
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.