Anselme v2.0.0-alpha rewrite

Woke up and felt like changing a couple things. It's actually been worked on for a while, little at a time...

The goal was to make the language and implementation much simpler. Well I don't know if it really ended up being simpler but it sure is more robust.

Main changes:
* proper first class functions and closures supports! proper scoping rules! no more namespace shenanigans!
* everything is an expression, no more statements! make the implementation both simpler and more complex, but it's much more consistent now! the syntax has massively changed as a result though.
* much more organized and easy to modify codebase: one file for each AST node, no more random fields or behavior set by some random node exceptionally, everything should now follow the same API defined in ast.abstract.Node

Every foundational feature should be implemented right now. The vast majority of things that were possible in v2 are possible now; some things aren't, but that's usually because v2 is a bit more sane.
The main missing things before a proper release are tests and documentation. There's a few other things that might be implemented later, see the ideas.md file.

parser/code_to_tree.lua

@@ -0,0 +1,65 @@
+--- transform raw code string into a nested tree of lines
+
+local Source = require("parser.Source")
+
+local function indented_to_tree(indented)
+	local tree = {}
+	local current_parent = tree
+	local current_level = 0
+	local last_line_empty = nil
+
+	for _, l in ipairs(indented) do
+		-- indentation of empty line is determined using the next line
+		-- (consecutive empty lines are merged into one)
+		if l.content == "" then
+			last_line_empty = l
+		else
+			-- raise indentation
+			if l.level > current_level then
+				if #current_parent == 0 then -- can't add children to nil
+					error(("invalid indentation; at %s"):format(l.source))
+				end
+				current_parent = current_parent[#current_parent]
+				current_level = l.level
+			-- lower indentation
+			elseif l.level < current_level then
+				current_parent = tree
+				current_level = 0
+				while current_level < l.level do -- find correct level starting back from the root
+					current_parent = current_parent[#current_parent]
+					current_level = current_parent[1].level
+				end
+				if current_level ~= l.level then
+					error(("invalid indentation; at %s"):format(l.source))
+				end
+			end
+			-- add line
+			if last_line_empty then
+				last_line_empty.level = current_level
+				table.insert(current_parent, last_line_empty)
+				last_line_empty = nil
+			end
+			table.insert(current_parent, l)
+		end
+	end
+
+	return tree
+end
+
+local function code_to_indented(code, source_name)
+	local indented = {}
+
+	local i = 1
+	for line in (code.."\n"):gmatch("(.-)\n") do
+		local indent, rem = line:match("^(%s*)(.-)$")
+		local indent_len = utf8.len(indent)
+		table.insert(indented, { level = indent_len, content = rem, source = Source:new(source_name, i, 1+indent_len) })
+		i = i + 1
+	end
+
+	return indented
+end
+
+return function(code, source_name)
+	return indented_to_tree(code_to_indented(code, source_name or "?"))
+end