util: prefer "bytesio" to "stringio"

util: prefer "bytesio" to "stringio"

The io.BytesIO and io.StringIO types enforce the type of
data being operated on. On Python 2, we use cStringIO.StringIO(),
which is lax about mixing types. On Python 3, we actually use
io.BytesIO. Ideally, we'd use io.BytesIO on Python 2. But I believe
its performance is poor compared to cString.StringIO().

Anyway, we canonically define our pycompat type as "stringio."
That name is misleading, especially on Python 3.

This commit renames the canonical symbols to "bytesio."
"stringio" is preserved as an alias for API compatibility. There
are a lot of callers in the repo and I hesitate to take away the
old name. I also don't feel like changing everything at this time.
But at least new callers can use a "proper" name.

Differential Revision: https://phab.mercurial-scm.org/D2868

--- a/mercurial/pure/mpatch.py	Wed Mar 14 14:05:45 2018 -0400
+++ b/mercurial/pure/mpatch.py	Wed Mar 14 11:52:35 2018 -0700
@@ -10,7 +10,7 @@
 import struct
 
 from .. import pycompat
-stringio = pycompat.stringio
+stringio = pycompat.bytesio
 
 class mpatchError(Exception):
     """error raised when a delta cannot be decoded

--- a/mercurial/pure/parsers.py	Wed Mar 14 14:05:45 2018 -0400
+++ b/mercurial/pure/parsers.py	Wed Mar 14 11:52:35 2018 -0700
@@ -12,7 +12,7 @@
 
 from ..node import nullid
 from .. import pycompat
-stringio = pycompat.stringio
+stringio = pycompat.bytesio
 
 
 _pack = struct.pack

--- a/mercurial/pycompat.py	Wed Mar 14 14:05:45 2018 -0400
+++ b/mercurial/pycompat.py	Wed Mar 14 11:52:35 2018 -0700
@@ -64,7 +64,9 @@
     sysexecutable = sys.executable
     if sysexecutable:
         sysexecutable = os.fsencode(sysexecutable)
-    stringio = io.BytesIO
+    bytesio = io.BytesIO
+    # TODO deprecate stringio name, as it is a lie on Python 3.
+    stringio = bytesio
 
     def maplist(*args):
         return list(map(*args))
@@ -343,7 +345,8 @@
     getcwd = os.getcwd
     sysexecutable = sys.executable
     shlexsplit = shlex.split
-    stringio = cStringIO.StringIO
+    bytesio = cStringIO.StringIO
+    stringio = bytesio
     maplist = map
     ziplist = zip
     rawinput = raw_input

--- a/mercurial/util.py	Wed Mar 14 14:05:45 2018 -0400
+++ b/mercurial/util.py	Wed Mar 14 11:52:35 2018 -0700
@@ -71,7 +71,9 @@
 stderr = pycompat.stderr
 stdin = pycompat.stdin
 stdout = pycompat.stdout
-stringio = pycompat.stringio
+bytesio = pycompat.bytesio
+# TODO deprecate stringio name, as it is a lie on Python 3.
+stringio = bytesio
 xmlrpclib = pycompat.xmlrpclib
 
 httpserver = urllibcompat.httpserver