Add {let+,and+} applicative syntax

[craigfe]

A follow up to #775 proposing Syntax.(( let+ ), ( and+ )) operators for the Lwt and Lwt_result modules.

When working with Lwt, I find myself using >|= often enough that a switch to open Lwt.Syntax would be frustrating without an equivalent syntactic form. As a sample from the mirage/irmin codebase:

ᐅ git grep ">>=" | wc -l
1480

ᐅ git grep ">|=" | wc -l
347

The proposed ( and+ ) operations behave the same as ( and* ): I find it convenient / aesthetic to use the corresponding and<op> for whichever let<op> I'm using. We've been using similar aliases in OCurrent syntax.

[aantron]

Thanks! I'll merge this tomorrow and release 5.3.0 with it.

[aantron]

This is now out in Lwt 5.3.0.