lang: interfaces, funcs: Port Func API to new Stream signature

This removes the `Close() error` and replaces it with a more modern
Stream API that takes a context. This removes boilerplate and makes
integration with concurrent code easier. The only downside is that there
isn't an explicit cleanup step, but only one function was even using
that and it was possible to switch it to a defer in Stream.

This also renames the functions from polyfunc to just func which we
determine by API not naming.

lang/interfaces/func.go

@@ -18,6 +18,7 @@
 package interfaces
 
 import (
+	"context"
 	"fmt"
 	"strings"
 
@@ -69,9 +70,18 @@ type Func interface {
 	// not known yet. This is because the Info method might be called
 	// speculatively to aid in type unification.
 	Info() *Info
+
+	// Init passes some important values and references to the function.
 	Init(*Init) error
-	Stream() error
-	Close() error
+
+	// Stream is the mainloop of the function. It reads and writes from
+	// channels to return the changing values that this func has over time.
+	// It should shutdown and cleanup when the input context is cancelled.
+	// It must not exit before any goroutines it spawned have terminated.
+	// It must close the Output chan if it's done sending new values out. It
+	// must send at least one value, or return an error. It may also return
+	// an error at anytime if it can't continue.
+	Stream(context.Context) error
 }
 
 // PolyFunc is an interface for functions which are statically polymorphic. In