all: remove support for Go 1.11 and 1.12

Go 1.13 has some important improvements that we would like to use, such
as the new integer literals and `errors.Is` which both arrived in Go
1.13.

src/runtime/strings_go111.go

@@ -1,20 +0,0 @@
-// +build !go1.12
-
-package runtime
-
-import "internal/bytealg"
-
-// The following functions provide compatibility with Go 1.11.
-// See the following:
-// https://github.com/tinygo-org/tinygo/issues/351
-// https://github.com/golang/go/commit/ad4a58e31501bce5de2aad90a620eaecdc1eecb8
-
-//go:linkname indexByte strings.IndexByte
-func indexByte(s string, c byte) int {
-	return bytealg.IndexByteString(s, c)
-}
-
-//go:linkname bytesEqual bytes.Equal
-func bytesEqual(a, b []byte) bool {
-	return bytealg.Equal(a, b)
-}