Rework syscall ABI.

Syscalls now accept two parameters, allowing for things like "int count = read(buf, len)"
Rather than providing safe signatures for syscalls, the user is now given helper functions to safely parse incoming values, c-strings and slices.

apps/sketch/sketch.c

@@ -9,6 +9,7 @@ uint32_t count;
 
 bool loop(void) {
     printdec(count);
+    println("");
     if (count++ >= 10) {
         return false;
     } else {