feat: update stdlib mapping for go1.15

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

stdlib/go1_15_crypto_sha256.go

@@ -0,0 +1,25 @@
+// Code generated by 'github.com/containous/yaegi/extract crypto/sha256'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"crypto/sha256"
+	"go/constant"
+	"go/token"
+	"reflect"
+)
+
+func init() {
+	Symbols["crypto/sha256"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"BlockSize": reflect.ValueOf(constant.MakeFromLiteral("64", token.INT, 0)),
+		"New":       reflect.ValueOf(sha256.New),
+		"New224":    reflect.ValueOf(sha256.New224),
+		"Size":      reflect.ValueOf(constant.MakeFromLiteral("32", token.INT, 0)),
+		"Size224":   reflect.ValueOf(constant.MakeFromLiteral("28", token.INT, 0)),
+		"Sum224":    reflect.ValueOf(sha256.Sum224),
+		"Sum256":    reflect.ValueOf(sha256.Sum256),
+	}
+}