Better core component props support

[myitcv]

Via an improved reactGen, we now support props type declaration "explosion"

Prior to this PR, the pattern for writing core React component props (or indeed any props type that embed another struct type) was:

// PPropsDef defines the properties for the <p> element
type PPropsDef struct {
	*BasicHTMLElement
}

// PProps creates a new instance of <p> properties, mutating the props
// by the provided initiapser
func PProps(f func(p *PPropsDef)) *PPropsDef {
	res := &PPropsDef{
		BasicHTMLElement: newBasicHTMLElement(),
	}
	f(res)
	return res
}

This pattern was necessary so that one could easily provide a field value without having to initialise deeply nested embedded struct values.

This callback pattern however made code much harder to write and read. Painful in fact.

Props type template explosion

reactGen now knows how to "explode" template struct type declarations for props types:

// _PProps are the props for a <div> component
type _PProps struct {
	*BasicHTMLElement
}

gets exploded into:

// PProps are the props for a <div> component
type PProps struct {
	ID                      string
	Key                     string
	ClassName               string
	Role                    string
	OnChange                func(e *SyntheticEvent)
	OnClick                 func(e *SyntheticMouseEvent)
	DangerouslySetInnerHTML *DangerousInnerHTMLDef
}

This allows a far more natural instantiation of components.

Compare how we used to have to do things

r.Div(
        r.DivProps(func(dp *r.DivPropsDef) {
                dp.ClassName = "row"
        }),
        r.Div(
                r.DivProps(func(dp *r.DivPropsDef) {
                        dp.ClassName = "col-md-8"
                }),
                r.Div(
                        r.DivProps(func(dp *r.DivPropsDef) {
                                dp.ClassName = "panel panel-default with-nav-tabs"
                        }),
                        r.Div(
                                r.DivProps(func(dp *r.DivPropsDef) {
                                        dp.ClassName = "panel-heading"
                                }),
                                r.Ul(
                                        r.UlProps(func(ulp *r.UlPropsDef) {
                                                ulp.ClassName = "nav nav-tabs"
                                        }),

                                        p.buildExampleNavTab(key, tabGo, "GopherJS"),
                                        p.buildExampleNavTab(key, tabJsx, "JSX"),
                                ),
                        ),
                        r.Div(
                                r.DivProps(func(dp *r.DivPropsDef) {
                                        dp.ClassName = "panel-body"
                                }),
                                r.Pre(nil, code),
                        ),
                ),
        ),
        r.Div(
                r.DivProps(func(dp *r.DivPropsDef) {
                        dp.ClassName = "col-md-4"
                }),
                plainPanel(elem),
        ),
)

with the new world post this PR:

r.Div(&r.DivProps{ClassName: "row"},
        r.Div(&r.DivProps{ClassName: "col-md-8"},
                r.Div(&r.DivProps{ClassName: "panel panel-default with-nav-tabs"},
                        r.Div(&r.DivProps{ClassName: "panel-heading"},
                                r.Ul(
                                        &r.UlProps{ClassName: "nav nav-tabs"},

                                        p.buildExampleNavTab(key, tabGo, "GopherJS"),
                                        p.buildExampleNavTab(key, tabJsx, "JSX"),
                                ),
                        ),
                        r.Div(&r.DivProps{ClassName: "panel-body"},
                                r.Pre(nil, code),
                        ),
                ),
        ),
        r.Div(&r.DivProps{ClassName: "col-md-4"},
                plainPanel(elem),
        ),
)