Inline max_slice_length
Inline max_slice_length

Note that where we previously ran `max_slice_len` with input having not
only matrix.heads() but also v.head(). Now we run it on matrix.heads()
only, but also take into account the currently processed constructor.
This preserves behavior since `pat_constructors` returns only one
constructor in the case that is of interest for us.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust