Misc Export fixes

- Temporarily disable export of base.cue as it causes merge errors at
  the end of compute
- Fixes for JSON export for Scalar and Lists
- Add YAML export
- Removed BOOL and NUMBER support, using JSON for now
- Re-enabled all export tests

Fixes #36

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

examples/tests/export/float/main.cue

@@ -0,0 +1,25 @@
+package testing
+
+test: {
+	float
+
+	#dagger: compute: [
+		{
+			do:  "fetch-container"
+			ref: "alpine"
+		},
+		{
+			do: "exec"
+			args: ["sh", "-c", """
+				echo -123.5 > /tmp/out
+				""",
+			]
+		},
+		{
+			do: "export"
+			// Source path in the container
+			source: "/tmp/out"
+			format: "json"
+		},
+	]
+}