Moved the library file to src/.

I get this error if I try to compile a file with -prof:

````
% ghc --make -prof profile.hs
[1 of 2] Compiling Falling          ( Falling.hs, Falling.o )
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Falling.hs:1:1:
    Dynamic linking required, but this is a non-standard build (eg. prof).
    You need to build the program twice: once the normal way, and then
    in the desired way using -osuf to set the object file suffix.
````

lispy on #haskell said to try putting the library in its own thing. It
fixes it! This is terrible!
Small program for profiling / benchmarking.
Make `Vector`'s fields strict.

I profiled and this shaved off almost half a second !!!
Type signature for `update`.

I guess this got left out.
Made Particle's fields strict.
Use a {-# SPECIALIZE ... #-} pragma for `update`.

This improves performance pretty dramatically for `profile 1000 100`.