Lwt: Update comments at the top of all code files

[aantron]

The first three lines of .mli and .ml files in Lwt currently look like this (example):

(* Lightweight thread library for OCaml
 * http://www.ocsigen.org/lwt
 * (Module|Interface) Lwt

we want them to look like this (example):

(* OCaml promise library
 * http://www.ocsigen.org/lwt

• "Lightweight thread" terminology is deprecated. We especially want to avoid it in .mli files, since people read those – though most people probably don't care about the file headers.
• Repeating the module name (which is based on the file name) in the header is redundant and fragile.

The header for src/core/lwt.ml is already being changed in #354, but the rest need to be done.

[jsthomas]

I'll work on this issue.

[aantron]

Done in #376. Thanks @jsthomas!