Remove compat layer and use generics directly

Backwards compatibility is not required since moving to v2.

nanotime_unsafe.go

@@ -0,0 +1,12 @@
+//go:build !purego && !appengine && !js
+
+// This file contains the implementation of nanotime using runtime.nanotime.
+
+package puddle
+
+import "unsafe"
+
+var _ = unsafe.Sizeof(0)
+
+//go:linkname nanotime runtime.nanotime
+func nanotime() int64