Pretty-printing of the * kind

Before this patch, GHC always printed the * kind unparenthesized.

This led to two issues:

1. Sometimes GHC printed invalid or incorrect code. For example, GHC would print type F @* x = x when it meant to print type F @(*) x = x. In the former case, instead of a kind application we were getting a type operator (@*).

2. Sometimes GHC printed kinds that were correct but hard to read. Should Either * Int be read as Either (*) Int or as (*) Either Int? This depends on whether -XStarIsType is enabled, but it would be easier if we didn't have to check for the flag when reading the code.

We can solve both problems by assigning * a different precedence. Note that Haskell98 kinds are not affected:

((* -> *) -> *) -> * does not become (((*) -> (*)) -> (*)) -> (*)

The parentheses are added when * is used in a function argument position:

• F * * * becomes F (*) (*) (*)
• F A * B becomes F A (*) B
• Proxy * becomes Proxy (*)
• a * -> * becomes a (*) -> *