Use custom dispatcher for Number.prototype routines (#2968)

Binary size gain:
     - Intel: ~630B (gcc-7.3)
     - Arm: ~280B (arm-linux-gnueabi-gcc-7.3)

Futher improvements:
 - Separate toFixed, toExponential, toPrecision common part into a helper function

Co-authored-by: Gabor Loki loki@inf.u-szeged.hu
JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

Author: rerobika