api: remove toLString()

There is no reason to have both toString() and toLString() when in Zig
we can get the length encoded in the slice type. Perhaps someone in the
future will raise a reason to have a separation with these functions. If
so, it is trivial to bring it back.

src/ziglua.zig

@@ -859,10 +859,8 @@ pub const Lua = struct {
         } else return c.lua_tointegerx(lua.state, index, null);
     }
 
-    /// Converts the Lua value at the given `index` to a C string
-    pub fn toLString(lua: *Lua, index: i32, length: ?*usize) [*]const u8 {
-        return c.lua_tolstring(lua.state, index, length);
-    }
+    // No need to have both toLString and toString for a Zig API
+    // pub fn toLString(lua: *Lua, index: i32) []const u8 { ... }
 
     /// Equivalent to toNumberX with is_num set to null
     pub fn toNumber(lua: *Lua, index: i32) Number {
@@ -886,9 +884,14 @@ pub const Lua = struct {
         return c.lua_topointer(lua.state, index);
     }
 
-    /// Equivalent to toLString with len equal to null
-    pub fn toString(lua: *Lua, index: i32) [*]const u8 {
-        return lua.toLString(index, null);
+    /// Converts the Lua value at the given `index` to a zero-terminated slice (string)
+    /// Returns null if the value was not a string or number
+    /// If the value was a number the actual value in the stack will be changed to a string
+    pub fn toString(lua: *Lua, index: i32) ?[:0]const u8 {
+        var length: usize = undefined;
+        if (c.lua_tolstring(lua.state, index, &length)) |str| {
+            return str[0..length :0];
+        } else return null;
     }
 
     /// Converts the value at the given `index` to a Lua thread (wrapped with a `Lua` struct)