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.

vec.c

@@ -18,15 +18,39 @@
 
 #include "./alisp.h"
 
+void vec_init(vec_t *vec, u64 size)
+{
+  memset(vec, 0, sizeof(*vec));
+  if (!vec)
+    return;
+  else if (size <= VEC_INLINE_CAPACITY)
+  {
+    vec->is_inlined = 1;
+    vec->capacity   = VEC_INLINE_CAPACITY;
+    vec->ptr        = NULL;
+  }
+  else
+  {
+    vec->is_inlined = 0;
+    vec->capacity   = size;
+    vec->ptr        = calloc(1, vec->capacity);
+  }
+}
+
 void vec_free(vec_t *vec)
 {
   if (!vec)
     return;
-  if (vec->data)
-    free(vec->data);
+  if (!vec->is_inlined && vec->ptr)
+    free(vec->ptr);
   memset(vec, 0, sizeof(*vec));
 }
 
+void *vec_data(vec_t *vec)
+{
+  return vec->is_inlined ? vec->inlined : vec->ptr;
+}
+
 void vec_ensure_free(vec_t *vec, u64 size)
 {
   if (!vec)
@@ -34,7 +58,21 @@ void vec_ensure_free(vec_t *vec, u64 size)
   if (vec->capacity - vec->size < size)
   {
     vec->capacity = MAX(vec->capacity * VEC_MULT, vec->size + size);
-    vec->data     = realloc(vec->data, vec->capacity);
+    if (vec->is_inlined)
+    {
+      // If we're inlined, we need to allocate on the heap now.  So let's copy
+      // vec->inlined over to vec->ptr, then turn off inlining.
+
+      // We need to do a two-way swap since vec->ptr and vec->inlined are taking
+      // up the same space.
+      u8 buffer[VEC_INLINE_CAPACITY];
+      memcpy(buffer, vec->inlined, vec->size);
+      vec->ptr = calloc(1, vec->capacity);
+      memcpy(vec->ptr, buffer, vec->size);
+      vec->is_inlined = 0;
+    }
+    else
+      vec->ptr = realloc(vec->ptr, vec->capacity);
   }
 }
 
@@ -43,7 +81,7 @@ void vec_append(vec_t *vec, void *ptr, u64 size)
   if (!vec)
     return;
   vec_ensure_free(vec, size);
-  memcpy(vec->data + vec->size, ptr, size);
+  memcpy(vec_data(vec) + vec->size, ptr, size);
   vec->size += size;
 }
 
@@ -51,7 +89,6 @@ void vec_clone(vec_t *dest, vec_t *src)
 {
   if (!src || !dest)
     return;
-  dest       = src;
-  dest->data = calloc(1, dest->capacity);
-  memcpy(dest->data, src->data, src->size);
+  vec_init(dest, src->capacity);
+  memcpy(vec_data(dest), vec_data(src), src->size);
 }