Add "Memory" tab to lg

Add "Memory" tab to lg

shell_global_get_memory_info() is a new function which extracts a few
global counters we have already, namely glibc's mallinfo, spidermonkey's
JSGC_BYTES, and gjs' counters for boxed/object/etc wrappers.

There is some slight overlap with perf; ultimately though I'd
like this function to do some more extensive analysis, so it wouldn't
be quite the same.

perf is going to be mainly concerned with how big the whole process
over time is; memory_info is for debugging memory leaks.

https://bugzilla.gnome.org/show_bug.cgi?id=650692