Refactor vectors to SBO, removing inlined entirely.

Avoid 2 levels of indirection, and having to allocate twice for small
payloads, by having an inlined array on the vector directly!
Beautiful and simple.

Required a bit of refactoring around the board, but overall the result
makes me feel happier.

build.sh

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