Currently, Tilemaker uses member functions for interop:
```lua
function node_function(node)
node:Layer(...)
```
This PR changes Tilemaker to use global functions:
```lua
function node_function()
Layer(...)
```
The chief rationale is performance. Every member function call needs to
push an extra pointer onto the stack when crossing the Lua/C++ boundary.
Kaguya serializes this pointer as a Lua userdata. That means every
call into Lua has to malloc some memory, and every call back from Lua
has to dereference through this pointer.
And there are a lot of calls! For OMT on the GB extract, I counted
~1.4B calls from Lua into C++.
A secondary rationale is that a global function is a bit more honest.
A user might believe that this is currently permissible:
```lua
last_node = nil
function node_function(node)
if last_node ~= nil
-- do something with last_node
end
-- save the current node for later, for some reason
last_node = node
```
But in reality, the OSM objects we pass into Lua don't behave quite
like Lua objects. They're backed by OsmLuaProcessing, who will move
on, invalidating whatever the user thinks they've got a reference to.
This PR has a noticeable decrease in reading time for me, measured
on the OMT profile for GB, on a 16-core computer:
Before:
```
real 1m28.230s
user 19m30.281s
sys 0m29.610s
```
After:
```
real 1m21.728s
user 17m27.150s
sys 0m32.668s
```
The tradeoffs:
- anyone with a custom Lua profile will need to update it, although the
changes are fairly mechanical
- Tilemaker now reserves several functions in the global namespace,
causing the potential for conflicts
Richard Fairhurst