Reinstate low priority benchmarks and docs.

Author: peterhinch

FASTPOLL.md

@@ -49,7 +49,7 @@ module documented [here](./lowpower/README.md).
   2.2 [Timing accuracy](./FASTPOLL.md#22-timing-accuracy)  
   2.3 [Polling in uasyncio](./FASTPOLL.md#23-polling-in-usayncio)  
  3. [The modified version](./FASTPOLL.md#3-the-modified-version)  
-  3.1 [Fast I/O](./FASTPOLL.md#31-fast-I/O)  
+  3.1 [Fast IO](./FASTPOLL.md#31-fast-IO)  
   3.2 [Low Priority](./FASTPOLL.md#32-low-priority)  
   3.3 [Other Features](./FASTPOLL.md#33-other-features)  
   3.4 [Low priority yield](./FASTPOLL.md#34-low-priority-yield)  
@@ -260,7 +260,9 @@ Arguments to `get_event_loop()`:
  3. `ioq_len=0` Length of I/O queue. Default: no queue is created.
  4. `lp_len=0` Length of low priority queue. Default: no queue.
 
-## 3.1 Fast I/O
+###### [Contents](./FASTPOLL.md#contents)
+
+## 3.1 Fast IO
 
 Device drivers which are to be capable of running at high priority should be
 written to use stream I/O: see
@@ -275,6 +277,8 @@ This behaviour may be desired where short bursts of fast data are handled.
 Otherwise drivers of such hardware should be designed to avoid hogging, using
 techniques like buffering or timing.
 
+###### [Contents](./FASTPOLL.md#contents)
+
 ## 3.2 Low Priority
 
 The low priority solution is based on the notion of "after" implying a time
@@ -296,6 +300,8 @@ It adds the following event loop methods:
 
 See [Low priority callbacks](./FASTPOLL.md#35-low-priority-callbacks)
 
+###### [Contents](./FASTPOLL.md#contents)
+
 ## 3.3 Other Features
 
 The version has a `version` variable containing 'fast_io'. This enables the
@@ -321,6 +327,9 @@ bar = Bar()  # Constructor calls get_event_loop()
 # and renders these args inoperative
 loop = asyncio.get_event_loop(runq_len=40, waitq_len=40)
 ```
+
+###### [Contents](./FASTPOLL.md#contents)
+
 ## 3.4 Low priority yield
 
 Consider this code fragment: