More modularization.

--HG--
rename : src/Main.hs => src/Sesyrel.hs
rename : src/Elimination.hs => src/Sesyrel/Elimination.hs
rename : src/Expression.hs => src/Sesyrel/Expression/Base.hs
rename : src/Ratio.hs => src/Sesyrel/Expression/Ratio.hs