Speed up NelderMead `centroid`.
Speed up NelderMead `centroid`.

This significantly cuts runtime and allocations. In my simple use-case
program this cut off quarter of the running program time and 20% of
allocations. Basic idea is simple: don't use `Simplex` as a container.
A very significant increase comes from using mutable vector rather
than applying `vadd` (now gone) in a loop. Good increase from using
`unsafeFreeze` over `freeze`: this is safe here as we safely `thaw`
first and don't use the mutable vector afterwards. An okay increase
from using tight `go` loop with length counting rather than doing
something like `take (VS.length p)` and using ST ref for index
tracking.

I have also added an export list and removed redundant imports for own
sanity and better inlining by GHC.

filopodia/open,diffusionkinetics/open,filopodia/open,diffusionkinetics/open,diffusionkinetics/open