Rewrite the table templates

Allow columns to stretch if they end up with wide content.

Apart from the hassle of having to manually update the calculations, having the
columns wide enough to hold the widest thing they might ever have leads to
tables with lots of whitespace in the results.

templating/build.py

@@ -93,6 +93,27 @@ def main(input_module, file_stream=None, out_dir=None, verbose=False):
 
         return '\n\n'.join(output_lines)
 
+    def fieldwidths(input, keys, defaults=[], default_width=15):
+        """
+        A template filter to help in the generation of tables.
+
+        Given a list of rows, returns a list giving the maximum length of the
+        values in each column.
+
+        :param list[dict[str, str]] input: a list of rows. Each row should be a
+           dict with the keys given in ``keys``.
+        :param list[str] keys: the keys corresponding to the table columns
+        :param list[int] defaults: for each column, the default column width.
+        :param int default_width: if ``defaults`` is shorter than ``keys``, this
+           will be used as a fallback
+        """
+        def colwidth(key, default):
+            return reduce(max, (len(row[key]) for row in input),
+                          default if default is not None else default_width)
+
+        results = map(colwidth, keys, defaults)
+        return results
+
     # make Jinja aware of the templates and filters
     env = Environment(
         loader=FileSystemLoader(in_mod.exports["templates"]),
@@ -102,6 +123,7 @@ def main(input_module, file_stream=None, out_dir=None, verbose=False):
     env.filters["indent"] = indent
     env.filters["indent_block"] = indent_block
     env.filters["wrap"] = wrap
+    env.filters["fieldwidths"] = fieldwidths
 
     # load up and parse the lowest single units possible: we don't know or care
     # which spec section will use it, we just need it there in memory for when