Type system for actions

[girvel]

There's an abstract "class" action.base, that provides methods :act and :is_available and may use :_act and :_is_available. Currently, actions are implemented through Table.extend with action.base as a second argument. This has a few downsides:

1. Generally, getmetatable(x) does type checking for x. Actions do not follow that convention.
2. Using Table.extend is not fully compatible with LuaLS; :is_available and :act have parameter types, underscore methods do not.
3. Action definitions break type definition convention (local methods = {} stuff)

Proposal:

1. Use metatable inheritance:

local methods = setmetatable({}, {__index = parent_mt.__index})

-- or maybe
local methods = Table.inherit(parent.mt)

2. action.base -> action.mt
3. Maybe Table.inherits_mt(x, mt) -> boolean to check inheritance

That's kind of OOP-ish, but otherwise seems to do the job.