[P0] Vector and tuple patterns in match/let silently return () instead of binding

[ecto]

The Pattern Matching guide shows vector destructuring in both
match and let:

[match pair
  #[0 _] "starts with zero"
  ...]

[let #[a b c] #[1 2 3]]
[println b]  ; "2"

Under loon run (0.6), these forms return () instead of matching
or binding. Only constructor patterns ([Foo x]), literal patterns,
wildcard, and plain binding work in match; vector and map
destructuring only work in lambda parameters ([fn [[a b]] x]).

Acceptance criteria

• Implement #[...] patterns in match and let so the guide samples
  run verbatim, or remove them from the guide and steer readers
  to lambda-param destructuring.
• Add tests for whichever direction is chosen.

Found while completing the hyperpolyglot ml column (clarkgrubb/hyperpolyglot#139).