[P1] assoc and update silently no-op on vectors (immutable index update unsupported)

[ecto]

assoc and update work on maps but silently return the original
vector when called with a vector + integer index:

[assoc {:a 1} :b 2]                ; {:b 2 :a 1}    -- works
[assoc #[1 2 3] 0 9]               ; #[1 2 3]       -- silent no-op
[update #[1 2 3] 0 [fn [x] 9]]     ; #[1 2 3]       -- silent no-op

There is no immutable index-update for vectors today, and the silent
no-op makes this look like the operation succeeded.

Acceptance criteria

• Either make assoc/update work on vectors with integer indices,
  returning a new vector — [assoc #[1 2 3] 0 9] -> #[9 2 3] — or
  raise a clear error when the wrong combination of types is passed.
• Add a set / assoc-in / equivalent if a different name is preferred.
• Document in ref/builtins.loon.