Add stage 06: Lua bootstrap

The goal of stage 06 is to try parse zig synax in lua. I pulled in
lpeglable 1.2.0 and parser-gen off github to get started. All of this
needs to be cleaned up rather soon.

Lua boostraps using tcc and musl from the previous stage. Since musl
0.6.0 doesn't support dynamic linking this build of lua doesn't support
shared libraries. I couldn't easily patch musl with dlopen and friends
so instead I link statically and call deps with c api.

06/lpeglabel/examples/expRec.lua

@@ -0,0 +1,128 @@
+local m = require"lpeglabel"
+local re = require"relabel"
+
+local labels = {
+  {"ExpTermFirst",  "expected an expression"},
+  {"ExpTermOp",   "expected a term after the operator"},
+  {"MisClose",  "missing a closing ')' after the expression"},
+}
+
+local function labelindex(labname)
+  for i, elem in ipairs(labels) do
+    if elem[1] == labname then
+      return i
+    end
+  end
+  error("could not find label: " .. labname)
+end
+
+local errors, subject
+
+local function expect(patt, labname)
+  local i = labelindex(labname)
+  return patt + m.T(i)
+end
+
+
+local num = m.R("09")^1 / tonumber
+local op = m.S("+-")
+
+local function compute(tokens)
+  local result = tokens[1]
+  for i = 2, #tokens, 2 do
+    if tokens[i] == '+' then
+      result = result + tokens[i+1]
+    elseif tokens[i] == '-' then
+      result = result - tokens[i+1]
+    else
+      error('unknown operation: ' .. tokens[i])
+    end
+  end
+  return result
+end
+
+local g = m.P {
+	"Exp",
+	Exp = m.Ct(m.V"OperandFirst" * (m.C(op) * m.V"Operand")^0) / compute,
+	OperandFirst = expect(m.V"Term", "ExpTermFirst"),
+	Operand = expect(m.V"Term", "ExpTermOp"),
+	Term = num + m.V"Group",
+	Group = "(" * m.V"Exp" * expect(")", "MisClose"),
+}
+
+function recorderror(pos, lab)
+	local line, col = re.calcline(subject, pos)
+	table.insert(errors, { line = line, col = col, msg = labels[lab][2] })
+end
+
+function record (labname)
+	return (m.Cp() * m.Cc(labelindex(labname))) / recorderror
+end
+
+function sync (p)
+	return (-p * m.P(1))^0
+end
+
+function defaultValue (p)
+	return p or m.Cc(1000) 
+end
+
+local grec = m.P {
+	"S",
+	S = m.Rec(m.V"A", m.V"ErrExpTermFirst", labelindex("ExpTermFirst")), -- default value is 0
+	A = m.Rec(m.V"Sg", m.V"ErrExpTermOp", labelindex("ExpTermOp")),
+	Sg = m.Rec(g, m.V"ErrMisClose", labelindex("MisClose")),
+	ErrExpTermFirst = record("ExpTermFirst") * sync(op + ")") * defaultValue(),
+	ErrExpTermOp = record("ExpTermOp") * sync(op + ")") * defaultValue(),
+	ErrMisClose = record("MisClose") * sync(m.P")") * defaultValue(m.P""),
+}
+               
+local function eval(input)
+	errors = {}
+	io.write("Input: ", input, "\n")
+	subject = input
+  local result, label, suffix = grec:match(input)
+  io.write("Syntactic errors found: " .. #errors, "\n")
+	if #errors > 0 then
+    local out = {}
+    for i, err in ipairs(errors) do
+      local pos = err.col
+      local msg = err.msg
+      table.insert(out, "syntax error: " .. msg .. " (at index " .. pos .. ")")
+    end
+    print(table.concat(out, "\n"))
+  end
+	io.write("Result = ")
+	return result	
+end
+
+print(eval "90-70-(5)+3")
+--> 20
+
+print(eval "15+")
+--> 2 + 0
+
+print(eval "-2")
+--> 0 - 2 
+
+print(eval "1+3+-9")
+--> 1 + 3 + [0] - 9
+
+print(eval "1+()3+")
+--> 1 + ([0]) [3 +] [0]
+
+print(eval "8-(2+)-5")
+--> 8 - (2 + [0]) - 5 
+
+print(eval "()")
+
+print(eval "")
+
+print(eval "1+()+")
+
+print(eval "1+(")
+
+print(eval "3)")
+
+print(eval "11+())3")
+