Externals: Add libfmt 5.3.0 to externals

Allows us to migrate off of printf specifiers and have more type-safe
formatting facilities. It also allows for custom type support as well.
fmt is also on track to have part of it standardized within C++2a, so
this will also lessen the transitional work necessary later on by
allowing new code to use it.

This simply adds the library but doesn't do anything with it yet.

Externals/fmt/support/update-coverity-branch.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python
+# Update the coverity branch from the master branch.
+# It is not done automatically because Coverity Scan limits
+# the number of submissions per day.
+
+from __future__ import print_function
+import shutil, tempfile
+from subprocess import check_output, STDOUT
+
+class Git:
+    def __init__(self, dir):
+        self.dir = dir
+
+    def __call__(self, *args):
+        output = check_output(['git'] + list(args), cwd=self.dir, stderr=STDOUT)
+        print(output)
+        return output
+
+dir = tempfile.mkdtemp()
+try:
+    git = Git(dir)
+    git('clone', '-b', 'coverity', 'git@github.com:fmtlib/fmt.git', dir)
+    output = git('merge', '-X', 'theirs', '--no-commit', 'origin/master')
+    if 'Fast-forward' not in output:
+        git('reset', 'HEAD', '.travis.yml')
+        git('checkout', '--', '.travis.yml')
+        git('commit', '-m', 'Update coverity branch')
+    git('push')
+finally:
+    shutil.rmtree(dir)