This is a particular case of Go append builtin where the array
type is inferred from the first argument. We detect using untyped
nil as argument, then we convert it to the array type prior to
proceeed further. It fixes further consistency checks and value
generation.

Fixes gnolang#1136.

Fixes also gnolang#1149.

Co-authored-by: Morgan <[email protected]>

This is done in ConverTo, so hopefully it works both at parsing
and during execution, and for composite types.

Some additional test cases have been added too.