func (fs *functionStore) makeAvailableScriggoFn(pkg *ast.Package, name string, fn *runtime.Function) {
	if fs.availableScriggoFuncs[pkg] == nil {
		fs.availableScriggoFuncs[pkg] = map[string]*runtime.Function{}
	}
	fs.availableScriggoFuncs[pkg][name] = fn
}
makeAvailableScriggoFn makes available the given function with the given name
in the pkg package, ensuring that such function can be later retrieved if it
is referenced in the Scriggo compiled code.
package compiler
import (
	"reflect"
	"github.com/open2b/scriggo/ast"
	"github.com/open2b/scriggo/internal/runtime"
)
type functionStore struct {
	emitter *emitter
	availableScriggoFuncs map[*ast.Package]map[string]*runtime.Function
	scriggoFuncIndexes map[*runtime.Function]map[*runtime.Function]int8
	predefFuncIndexes map[*runtime.Function]map[reflect.Value]int8
}
func newFunctionStore(emitter *emitter) *functionStore {
	return &functionStore{
		emitter:               emitter,
		availableScriggoFuncs: map[*ast.Package]map[string]*runtime.Function{},
		scriggoFuncIndexes:    map[*runtime.Function]map[*runtime.Function]int8{},
		predefFuncIndexes:     map[*runtime.Function]map[reflect.Value]int8{},
	}
}