alias `Infix` with `Syntax` everywhere, deprecate `Infix`

[c-cube]

seems like the trend is using Syntax (e.g in lwt), and it's more accurate than Infix for the new let operators.

[FardaleM]

In Lwt, the module Infix still exists and is for the infix operator like >>=. The module Syntax contains let* and other. Currently the monadic let bindings are in the module Infix, but we can have a separate module Syntax for them.

[c-cube]

good point.