Merge pull request #865 from qtxie/float

FEAT: handle special float literal 1.#NaN and +/-1.#INF.
LIB: added `dtoa` library.

TESTS: added tests for `dtoa` library.
Merge pull request #866 from qtxie/dtoa-library

LIB: added `dtoa` library.
FEAT: try best to make consistent results of `form-float`.

TESTS: added tests for float comparasion.
TESTS: added tests for arithmetic of float special values(NaN, INF).