Add `wml/merge_attributes()`.
Add `math` library, with `math/clamp()`.
modifications: Add terrain-cost effect handlers.

Add `modifications/effect_handlers.{movement,vision,jamming}_costs()`.
Add `modifications/effect_handlers.resistance()`.
Add `modifications/effect_handlers.defense()`.
Properly refer to “add-ons servers” in LICENSE.
Delimit a URL with ‘<…>’ rather than ‘[…]’.
Make `string` library load without `wesnoth`.
Add `string/strtod()`.
Delimit strings with ‘`’ in docs/string.md.

Prevents `<foo>` from being interpreted as an HTML start tag.
Note automatic `tostring`-ing in docs/string.md.

Note in docs/string.md that all lp8 `string` library functions pass
their `string` parameters through `tostring` before processing them.
Add `string/parse_wml_boolean()`.

Add `string/parse_wml_boolean()`, a function to parse a WML Boolean
value to a Lua Boolean value.
Add `string/parse_wml_value()`.

Add `string/parse_wml_value()`, a function to parse a WML value to its
Lua equivalent.
Add `string/pattern_escape()`.

Add `string/pattern_escape()`, a function for escaping strings to be
used in or as standard Lua string patterns.
Add `utils/to_boolean()`.
Add `string/is_identifier()`.