output: merge base/input/output into a single state.cue

Exporting base/input/output separately causes merge errors.
For instance, `foo: string | *"default foo"` gets serialized as
`{"foo":"default foo"}`, which will fail to merge if output contains
a different definition of `foo`.

This change temporarily merges the 3 streams together before
serializing.

Fixes #51

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

examples/tests/input/simple/main.cue

@@ -0,0 +1,27 @@
+package testing
+
+X1=in: string
+
+test: {
+	string
+
+	#dagger: compute: [
+		{
+			do:  "fetch-container"
+			ref: "alpine"
+		},
+		{
+			do: "exec"
+			args: ["sh", "-c", """
+				echo -n "received: \(X1)" > /out
+				"""]
+			// XXX Blocked by https://github.com/blocklayerhq/dagger/issues/19
+			dir: "/"
+		},
+		{
+			do:     "export"
+			source: "/out"
+			format: "string"
+		},
+	]
+}