Replaces all occurences of =/2 in the do block by fit/2

iex> import PatternMetonyms
...> fit do
...>   (Kernel.+(1) -> x) = 1
...>   (Kernel.-(1) -> y) = 41
...> end
...> x + y
42

iex> import PatternMetonyms
...> fit(do: (Kernel.+(1) -> x) = (Kernel.-(1) -> y) = 21)
...> x + y
42

=/2 equivalent to view, named form.

iex> import PatternMetonyms
...> pat = fn {:Just, x} -> x end
...> fit((pat.() -> n), {:Just, 3})
...> n + 1
4

Underscore prefixed variables are ignored, they are therefore unusable outside fit/2's first argument.

Because of the lack of operator with the same associativity and precedence as =/2, no operator form of fit/2 is planned. It would otherwise result in unintuitive behaviors.

Left nesting of fit/2 will result in a compilation error.