feat: update stdlib mapping for go1.15

- drop stdlib for go1.13
- generate stdlib for go1.15
-  update CI configuration.

stdlib/go1_15_go_doc.go

@@ -0,0 +1,37 @@
+// Code generated by 'github.com/containous/yaegi/extract go/doc'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"go/doc"
+	"reflect"
+)
+
+func init() {
+	Symbols["go/doc"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"AllDecls":        reflect.ValueOf(doc.AllDecls),
+		"AllMethods":      reflect.ValueOf(doc.AllMethods),
+		"Examples":        reflect.ValueOf(doc.Examples),
+		"IllegalPrefixes": reflect.ValueOf(&doc.IllegalPrefixes).Elem(),
+		"IsPredeclared":   reflect.ValueOf(doc.IsPredeclared),
+		"New":             reflect.ValueOf(doc.New),
+		"NewFromFiles":    reflect.ValueOf(doc.NewFromFiles),
+		"PreserveAST":     reflect.ValueOf(doc.PreserveAST),
+		"Synopsis":        reflect.ValueOf(doc.Synopsis),
+		"ToHTML":          reflect.ValueOf(doc.ToHTML),
+		"ToText":          reflect.ValueOf(doc.ToText),
+
+		// type definitions
+		"Example": reflect.ValueOf((*doc.Example)(nil)),
+		"Filter":  reflect.ValueOf((*doc.Filter)(nil)),
+		"Func":    reflect.ValueOf((*doc.Func)(nil)),
+		"Mode":    reflect.ValueOf((*doc.Mode)(nil)),
+		"Note":    reflect.ValueOf((*doc.Note)(nil)),
+		"Package": reflect.ValueOf((*doc.Package)(nil)),
+		"Type":    reflect.ValueOf((*doc.Type)(nil)),
+		"Value":   reflect.ValueOf((*doc.Value)(nil)),
+	}
+}