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/helloworld/helloworld.c

@@ -1,8 +1,6 @@
 #include "uvm32_target.h"
 
 void main(void) {
-    for (int i=0;i<10;i++) {
-        printdec(i);
-    }
+    println("Hello world");
 }