Support mapping to fields on sub structures for arbitrary levels.

[ekhabarov]

Source structure

type Source struct {
	I int `sts: "First.Second.Third.I"`
	S int `sts: "First.S"`
}

destination structure

type Dest struct {
	First struct {
		Second struct {
			Third struct {
				I int
				S string
			}
		}
		S string
	}
}

[tooolbox]

This is valuable.

Also does the tool support nested structs where the definitions of those structs are in other files in the same package, or other packages entirely? That would be valuable.