markused: fixing missing info tracking on walker

[felipensp]

Some fixes I've done detected by running --skip-unused on v.ui, vtl, vsl, ved etc.

• Make walker visit more nodes (e.g. {...foo})
• Handle function as global var
• Load embeded struct definition
• Load struct when used on []Struct
• Load struct when used on map[k]Struct

_{Huly®: V_0.6-21449}

[spytheman]

Can you please also add some isolated examples in vlib/v/tests/skip_unused/ ?
Having it work for bigger projects is nice, but the sources of those can change, and it will be nice to have samples, that will continue to test that new functionality.

[spytheman]

Excellent work.

[spytheman]

For me, ved compiles with -skip-unused on both master, and on this PR.

~/.vmodules/ui/examples/rectangles.v does not compile on both master and on this PR:

[spytheman]

~/.vmodules/vsl/examples/diff_example/ also compiled on both master and here: