Improve custom-operator precedences

[inamiy]

This is a pull-request for #9 to conform the same operator-precedences as following libraries:

• https://github.com/thoughtbot/Runes
• https://github.com/typelift/Operadics

As a side note,

• infix operator <|> { associativity right } was slightly faster than associativity left (interesting), but I decided to conform to current Haskell's infixl 3 <|> with precedence 125.
  • precedence 125 comes from middle value of infix 2 (precedence 120) and infix 4 (precedence 130) in typelift/Operadics.

[koher]

I'm not sure if we should introduce a new precedence 125 for <|> because someone who want to introduce a operator compatible with infixl 3 does not necessarily use 125 for it. It can be anything between 121 and 129. Introducing a new precedence can always make an incompatibility. So I hesitate it.

Using ?? as <|> make it incompatible with Haskell. However it keeps compatibilities in Swift. I think it depends on the purpose of TryParsec whether we should use ?? instead of <|>. If we want to provide TryParsec to people who are familiar with Haskell, it may be a problem. Otherwise, I think ?? is OK.

[inamiy]

Let's keep Haskell-style writing so that current codes stay beautiful with angle-brackets all over the place 😁

(As already discussed in #9, I still find ?? difficult to read because I normally use it as fromMaybe, and <|> truly represents "or" operator since | is inside.)

[koher]

Sorry for my late response💦

I see😉 I think consistency in codes is important too.

[inamiy]

I will close this old PR. This is now superseded by #15.