Add idea for "rcl build"

Thanks Tijmen for triggering this idea!

ideas/rcl_build.md

@@ -0,0 +1,38 @@
+# `rcl build`
+
+There is now the possibility to generate a depfile, and generate Ninja, etc.
+But the overhead is quite large. For a big repository it's fine, but if I just
+want to generate two very similar GitHub Actions files, it's overkill. I can
+split out the shared part and have 3 files + a shell script to write. Or I can
+have one file + shell script that runs `rcl query`, but I duplicate the keys
+between the shell script and the RCL file ...
+
+What if we could have a `build.rcl`, with this schema?
+
+    <top-level>: List[Target]
+
+    type Target = {
+      path: String,
+      format: String,
+      banner:? String,
+      value: Any,
+    }
+
+And a subcommand `rcl build` that would evaluate such a file, and for every
+target, write the value to the given path in the given format. Then for simple
+GitHub Actions, I would have only a *single* file, `build.rcl`, and I could
+define my targets in there!
+
+Or maybe the schema should be
+
+    <top-level>: Dict[String, Target]
+    type Target = {
+        format: String,
+        banner:? String,
+        value: Any,
+    }
+
+because then by construction you don't try to define the same file twice.
+
+Although, if we do a list, then we might also define includes, so you could have
+small `build.rcl` files scattered throughout a repo. Something to think about.

ideas/roadmap.md

@@ -9,7 +9,7 @@
  * Accept an expression through `--expr` for `rcl evaluate`.
  * Actually implement the `--in-place` for `rcl fmt`.
  * Add a `--follow` for `rcl fmt` that follows imports.
- * Add a way to output a depfile for Make/Ninja.
+ * Add an `rcl build` subcommand.
 
 ## Mid-term