Several subtle IODriver bug fixes

demos/interactionTrees/getLineAsync.tri

@@ -0,0 +1,25 @@
+-- Manual test for async getLine
+--
+-- Run with:
+--   nix run .# -- eval -f demos/async-getline-test.tri --io
+--
+-- Expected behaviour:
+--   1. You immediately see:
+--        Please enter your first name:
+--        (this printed before you typed anything)
+--        (this second line also printed before you typed anything)
+--   2. You type your name and press Enter.
+--   3. You see:
+--        Hello, <name>!
+
+!import "../lib/io.tri" !Local
+
+main = io <|
+  bind (fork getLine) (h :
+  bind (putStr "Please enter your first name: ") (_ :
+  bind (putStr "\n(this printed before you typed anything)\n") (_ :
+  bind (putStr "\n(this second line also printed before you typed anything)\n") (_ :
+  bind (await h) (name :
+  bind (putStr "Hello, ") (_ :
+  bind (putStr name) (_ :
+  putStr "!\n")))))))