docs: split monolithic core-library.markdown into per-class docs under doc/site/core/

doc/site/core/fiber.markdown

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+^title Fiber Class
+^category core
+
+A lightweight coroutine. [Here](../fibers.html) is a gentle introduction.
+
+### new **Fiber**(function)
+
+Creates a new fiber that executes `function` in a separate coroutine when the
+fiber is run. Does not immediately start running the fiber.
+
+    :::dart
+    var fiber = new Fiber {
+      IO.print("I won't get printed")
+    }
+
+### Fiber.**yield**
+
+Pauses the current fiber and transfers control to the parent fiber. "Parent"
+here means the last fiber that was started using `call` and not `run`.
+
+    :::dart
+    var fiber = new Fiber {
+      IO.print("Before yield")
+      Fiber.yield
+      IO.print("After yield")
+    }
+
+    fiber.call              // "Before yield"
+    IO.print("After call")  // "After call"
+    fiber.call              // "After yield"
+
+When resumed, the parent fiber's `call` method returns `null`.
+
+If a yielded fiber is resumed by calling `call()` or `run()` with an argument,
+`yield` returns that value.
+
+    :::dart
+    var fiber = new Fiber {
+      IO.print(Fiber.yield) // "value"
+    }
+
+    fiber.call          // Run until the first yield.
+    fiber.call("value") // Resume the fiber.
+
+If it was resumed by calling `call` or `run` with no argument, returns `null`.
+
+It is a runtime error to call this when there is no parent fiber to return to.
+
+    :::dart
+    Fiber.yield // ERROR
+
+    new Fiber {
+      Fiber.yield // ERROR
+    }.run
+
+### Fiber.**yield**(value)
+
+Similar to `Fiber.yield` but provides a value to return to the parent fiber's
+`call`.
+
+    :::dart
+    var fiber = new Fiber {
+      Fiber.yield("value")
+    }
+
+    IO.print(fiber.call) // "value"
+
+### **call**
+
+**TODO**
+
+### **call**(value)
+
+**TODO**
+
+### **isDone**
+
+Whether the fiber's main function has completed and the fiber can no longer be
+run. This returns `false` if the fiber is currently running or has yielded.
+
+### **run**
+
+**TODO**
+
+### **run**(value)
+
+**TODO**