codeprinters: fix Rational's printing for code (as fractions of doubles)

S(4)/3 * x -> (4.0/3.0)*x for ccode, not 4*x/3
ccode: print Rational numbers with long double
printing.rst: fix doctest failures for fcode
Merge branch 'master' into rationals-printing-in-code
Merge branch 'master' into rationals-printing-in-code
Merge branch 'master' into rationals-printing-in-code
Merge branch 'master' into rationals-printing-in-code
Merge branch 'master' into rationals-printing-in-code
Merge branch 'master' into rationals-printing-in-code
Merge pull request #1770 from skirpichev/rationals-printing-in-code

codeprinters: fix Rational's printing for code (as fractions of doubles)

S(4)/3 * x -> (4.0/3.0)*x for ccode, not 4*x/3