feat(sieve): add the remaining collection helpers and docs mentions

- Add TryMap, Filter, Reduce, KeyBy, GroupBy, Uniq, UniqBy, Partition
  and Find, plus the Set type built with SetOf (Has, Add, Slice).
- Add unit tests at 100% statement coverage and a runnable example for
  every function and Set method.
- Complete the module README: contracts, use cases, Pluck rationale,
  standard-library pointers and the full API reference.
- docs/architecture/introduction.md: add a Utilities row for sieve.
- docs/setup/coming-from-wintercms.md: map Laravel collection methods
  to sieve and narrow the not-provided row to the fluent collection
  object.

docs/architecture/introduction.md

@@ -30,6 +30,7 @@ Each framework module is one Go package under `modules/`, documented by its READ
 | Admin | [cabana](../../modules/cabana/README.md), [boardwalk](../../modules/boardwalk/README.md) |
 | Services | [phrasebook](../../modules/phrasebook/README.md), [postcard](../../modules/postcard/README.md), [conga](../../modules/conga/README.md), [lighthouse](../../modules/lighthouse/README.md), [flare](../../modules/flare/README.md), [sunscreen](../../modules/sunscreen/README.md) |
 | Console and tooling | [bonfire](../../modules/bonfire/README.md), [tide](../../modules/tide/README.md) |
+| Utilities | [sieve](../../modules/sieve/README.md) |
 
 ## What runs where