Add Optional type for filter pipes

Introduces `StaticType::Optional` to represent values that can be either
of a specific type `T` or `Void`. This is crucial for handling
situations in filter pipes where an expression might not always produce
a value.

The type checker now correctly deduces and propagates this `Optional`
type, and the pipeline operator (`pipe`) is updated to unwrap
`Optional(T)` results, yielding `T`. This ensures that the type system
accurately reflects the potential absence of values in intermediate
pipeline steps.

Also includes a minor rename in the tuple-struct example from `pipe` to
`p` for clarity, and registers the `streams` module.

examples/tuple-struct.myc

@@ -1,7 +1,7 @@
 ;; Benchmark: 890ns
 ;; Benchmark-Repeat: 2258
 (do
-     (def pipe (fn [conf]
+     (def p (fn [conf]
        (do
           (def [str s] conf)
           (def [f ss] s)
@@ -10,5 +10,5 @@
       )
      )
 
-     (pipe ["btc" [:close "cls"]])
+     (p ["btc" [:close "cls"]])
    )