Lua 55 support

[boolangery]

Adds Lua 5.5 support.

On the grammar side: global declarations (including global function and the global x * wildcard), named varargs (function f(...args)), an attribute on a whole local list, break as a statement of its own rather than part of retstat, and a stricter lexer for strings and escapes (\z, line continuations, nested long brackets).

While testing this I pulled the official Lua test suite in as a corpus (lua/lua at v5.5.1, 34 files) and ran every file through parse -> to_lua_source -> parse. All of them parsed, but the printer turned out to have a handful of bugs that only show up on real-world code:

• a trailing comment on a table field swallowed the field's comma, which quietly broke the rest of the table
• - -2 came out as --2, i.e. a comment
• block indentation was applied inside long-bracket strings and \z continuations, changing the string's actual value
• print"x" and f{...} were normalised to print("x") and f({...})

All four are fixed. The corpus round trip is now exact: same tree, every comment preserved, and printing is a fixpoint.