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/compute-powers.py

@@ -0,0 +1,53 @@
+#!/usr/bin/env python
+# Compute 10 ** exp with exp in the range [min_exponent, max_exponent] and print
+# normalized (with most-significant bit equal to 1) significands in hexadecimal.
+
+from __future__ import print_function
+
+min_exponent = -348
+max_exponent = 340
+step = 8
+significand_size = 64
+exp_offset = 2000
+
+class fp:
+    pass
+
+powers = []
+for i, exp in enumerate(range(min_exponent, max_exponent + 1, step)):
+    result = fp()
+    n = 10 ** exp if exp >= 0 else 2 ** exp_offset / 10 ** -exp
+    k = significand_size + 1
+    # Convert to binary and round.
+    binary = '{:b}'.format(n)
+    result.f = (int('{:0<{}}'.format(binary[:k], k), 2) + 1) / 2
+    result.e = len(binary) - (exp_offset if exp < 0 else 0) - significand_size
+    powers.append(result)
+    # Sanity check.
+    exp_offset10 = 400
+    actual = result.f * 10 ** exp_offset10
+    if result.e > 0:
+        actual *= 2 ** result.e
+    else:
+        for j in range(-result.e):
+            actual /= 2
+    expected = 10 ** (exp_offset10 + exp)
+    precision = len('{}'.format(expected)) - len('{}'.format(actual - expected))
+    if precision < 19:
+        print('low precision:', precision)
+        exit(1)
+
+print('Significands:', end='')
+for i, fp in enumerate(powers):
+    if i % 3 == 0:
+        print(end='\n ')
+    print(' {:0<#16x}'.format(fp.f, ), end=',')
+
+print('\n\nExponents:', end='')
+for i, fp in enumerate(powers):
+    if i % 11 == 0:
+        print(end='\n ')
+    print(' {:5}'.format(fp.e), end=',')
+
+print('\n\nMax exponent difference:',
+      max([x.e - powers[i - 1].e for i, x in enumerate(powers)][1:]))