sv: major refactor

- Internal data pointer is read only by default => any uses that
  require use of the pointer itself (freeing, mutating) must perform a
  cast.

- refactor tests to use some new sv macros and functions, and clean
  them up.

- slight cleanup of sv.c

include/alisp/sv.h

@@ -14,7 +14,7 @@
 typedef struct
 {
   u64 size;
-  char *data;
+  const char *data;
 } sv_t;
 
 // String view macro constructor