v: add map update-init syntax

[edam]

Add support for map update-init syntax to v:

foo := {
    ...base_map
    'a': 55
    'b': 66
}

which is functionally equivalent to:

mut foo := base_map.clone()
foo['a'] = 55
foo['b'] = 66

except that it yields an expression which does not have to be mutable.

This PR updates:

• ast, checker, parser and cgen
• adds new_map_update_init() to builtin/map.v to support cgen
• docs
• v fmt
• v tests & checker tests

[edam]

@spytheman are you able to retry the failing test? It seems unrelated?

[shove70]

@spytheman are you able to retry the failing test? It seems unrelated?

restarted.

[spytheman]

cmd/tools/vast/vast.v:1645 also needs updating, when adding new fields, that can carry meaningful information about the source. v ast -p file.v is very useful, for understanding the AST structure of a file.

[spytheman]

Excellent work.