feat: update stdlib mapping for go1.15

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

stdlib/go1_15_go_parser.go

@@ -0,0 +1,30 @@
+// Code generated by 'github.com/containous/yaegi/extract go/parser'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"go/parser"
+	"reflect"
+)
+
+func init() {
+	Symbols["go/parser"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"AllErrors":         reflect.ValueOf(parser.AllErrors),
+		"DeclarationErrors": reflect.ValueOf(parser.DeclarationErrors),
+		"ImportsOnly":       reflect.ValueOf(parser.ImportsOnly),
+		"PackageClauseOnly": reflect.ValueOf(parser.PackageClauseOnly),
+		"ParseComments":     reflect.ValueOf(parser.ParseComments),
+		"ParseDir":          reflect.ValueOf(parser.ParseDir),
+		"ParseExpr":         reflect.ValueOf(parser.ParseExpr),
+		"ParseExprFrom":     reflect.ValueOf(parser.ParseExprFrom),
+		"ParseFile":         reflect.ValueOf(parser.ParseFile),
+		"SpuriousErrors":    reflect.ValueOf(parser.SpuriousErrors),
+		"Trace":             reflect.ValueOf(parser.Trace),
+
+		// type definitions
+		"Mode": reflect.ValueOf((*parser.Mode)(nil)),
+	}
+}