Stable vector implementation

Stable vectors will be used in the lisp runtime to implement actual
vectors, instead of using the disgusting lvec trick.  This way we at
least can get attributes about the vector through one pointer hop.

build.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env sh
 
 CFLAGS="-Wall -Wextra -std=c11 -ggdb -fsanitize=address -fsanitize=undefined"
-SRC="ivec.c symtable.c tag.c constructor.c sys.c main.c"
+SRC="vec.c ivec.c symtable.c tag.c constructor.c sys.c main.c"
 OUT="alisp.out"
 
 set -xe