py: Add stream reading of n unicode chars; unicode support by default.

With unicode enabled, this patch allows reading a fixed number of
characters from text-mode streams; eg file.read(5) will read 5 unicode
chars, which can made of more than 5 bytes.

For an ASCII stream (ie no chars > 127) it only needs to do 1 read.  If
there are lots of non-ASCII chars in a stream, then it needs multiple
reads of the underlying object.

Adds a new test for this case.  Enables unicode support by default on
unix and stmhal ports.

tests/unicode/data/utf-8_2.txt

@@ -0,0 +1 @@
+aαbβcγdδ

tests/unicode/file2.py

@@ -0,0 +1,12 @@
+# test reading a given number of characters
+
+def do(mode):
+    f = open('unicode/data/utf-8_2.txt', mode)
+    print(f.read(1))
+    print(f.read(1))
+    print(f.read(2))
+    print(f.read(4))
+    f.close()
+
+do('rb')
+do('rt')