change fixity of the operators to be consistent with (<>) from base

(<>) from base is defined with infixr 6, so having local (<>) be infixl is rather confusing and makes concatenation of StringS inefficient. In addition, it is currently impossible to mix (<>) from base and (<+>) in the same statement without wrapping stuff in parentheses:

```haskell
λ> "foo" <> "bar:" <+> "baz"

<interactive>:20:1:
    Precedence parsing error
        cannot mix ‘<>’ [infixr 6] and ‘<+>’ [infixl 6] in the same infix expression
```