Replace FiniteMap and UniqFM with counterparts from containers.

The original interfaces are kept. There is small performance improvement:
- when compiling for five nofib, we get following speedups:
    Average                -----           -2.5%
    Average                -----           -0.6%
    Average                -----           -0.5%
    Average                -----           -5.5%
    Average                -----          -10.3%
- when compiling HPC ten times, we get:
    switches                          oldmaps   newmaps
    -O -fasm                          117.402s  116.081s (98.87%)
    -O -fasm -fregs-graph             119.993s  118.735s (98.95%)
    -O -fasm -fregs-iterative         120.191s  118.607s (98.68%)