fix(cil2smt2/basic.ml): increase precision in formatting float num
refactor(cil2smt2): move global vars upfront; clean up
feat(cil2smt2): add skeletal for call handler
feat(cil2smt2): filter out certain functions from translation

 - main, get_low_nbit
feat(cil2smt2): int variable ``x`` gets |sin(pi * x)| <= eps