Implement optimal parenthesis for ternary operator + add tests

[GreyCat]

Another piece of partially contributing to resolving https://github.com/kaitai-io/kaitai_struct/issues/69 — this one handles ternary operator:

• Add common doIfExp() implementation (+add extPrec propagation into it) into CommonOps, which does a good job at parenthesizing
  • This provides default implementation which is ok for most languages following popular condition ? ifTrue ? ifFalse rendering originating from C.
  • Remove individual copy-pasted implementations in specific languages where we can rely on common one.
  • Adjust Python rendering to follow the same correct strategy
  • Don't touch special ad hoc implementations (Lua, Go)
• Add/fix tests proving that it works in many contexts