Fix __PURE__ annotation placement

[kblcuk]

Currently there are some arrow functions that have /#PURE/
annotation before function call to identify that that function call is
side effect free.

However since this project is built to es5 target, which doesn't have
arrow functions support, the code is then translated into regular
function that loos something like this:

/** @internal */
export var flatMapNullable = function (F, M) {
    /*#__PURE__*/ return dual(3, function (self, f, onNullable) {
        return M.flatMap(self, liftNullable(F)(f, onNullable));
    });
};

However this makes some built tools (Vite (which uses Rollup under the
hood) in our case) unhappy, and build produces a lot of warnings about
that PURE annotation being in the wrong place.

Checking the Rollup docs [0], it seems that pure annotation should be
placed right before function invocation, so in this particular case
between return keyword and the actual function.

So this simply changes arrow functions in question to have explicit
return keyword and annotation before the function call, which leaves no
interpretation to ts compiler, and makes our code build process
warning-free.

This also potentially fixes #1916

[0] https://rollupjs.org/configuration-options/#pure

[gcanti]

Thank you @kblcuk, going to release this asap