Implement strings the way Go itself does

This makes string slicing cheap.

runtime/runtime.h

@@ -4,8 +4,8 @@
 #include <stdint.h>
 
 typedef struct {
-	uint32_t len;   // TODO: size_t (or, let max string size depend on target/flag)
-	uint8_t  buf[]; // variable size
+	uint32_t len;  // TODO: size_t (or, let max string size depend on target/flag)
+	uint8_t  *buf; // points to string buffer itself
 } string_t;
 
 typedef int32_t intgo_t; // may be 64-bit