Updating extlib to 1.7.8

[Andy Li <andy@onthewings.net>]

Hi Stéphane,

I've pushed to the extlib salsa repo with an update to the latest
upstream version 1.7.8.

The upstream changelog mentioned 2 breaking changes:
* breaking change: ExtList.find_map type updated to match stdlib
(following deprecation in previous release)
* breaking change: minimal (recommended) build of extlib now excludes
Base64 module

I've checked the build-rdeps.
I will handle haxe - its latest release is compatible with extlib 1.7.8.
nurpawiki and virt-top use ExtList, but without usages of find_map.
dose3 is compatible with extlib 1.7.8, according to its opam file. It
now uses Base64 from the ocaml-base64 lib.

Would you review and upload?

Best regards,
Andy