Upgrade to Duktape 2.5.0

[wyozi]

I upgraded Duktape to 2.5.0 to see if it would fix a SIGSEGV: invalid memory reference (spoiler: it didn't)

But maybe Duktape can be upgraded anyway, unless there is some reason not to? I downloaded duktape-2.5.0.tar.xz from https://duktape.org/download.html and replaced existing files. All tests seem to pass ok.

I also added instructions for future Duktape upgrades and note about ffi-gen workaround I had to do to get it to compile on OS X.

Btw I didn't see anything too impactful in Duktape changelogs, but Add "globalThis" binding, enabled by default. This replaces the previous "global" binding (which was disabled by default) to match the revised "global" specification. in 2.5 might be a breaking change? It could be disabled by default, although I would be happy to have it on by default.