Fix docs for classes with constructors (#174)

Avoid the null terminator that function signature includes which any C
program will interpret as end of string.

---------

Co-authored-by: Nicholas Gates <[email protected]>

example/buffers.pyi

@@ -3,5 +3,9 @@ from __future__ import annotations
 def sum(buf, /): ...
 
 class ConstantBuffer:
+    """
+    A class implementing a buffer protocol
+    """
+
     def __init__(elem, length, /):
         pass

example/iterators.pyi

@@ -1,6 +1,10 @@
 from __future__ import annotations
 
 class Range:
+    """
+    An example of iterable class
+    """
+
     def __init__(lower, upper, step, /):
         pass
     def __iter__(self, /):

pydust/src/functions.zig

@@ -432,6 +432,7 @@ fn sigSize(comptime sig: Signature) usize {
     var argSize: u64 = sig.name.len;
     // Count the size of the output string
     for (args) |arg| {
+        // +2 for ", "
         argSize += arg.len + 2;
     }
 
@@ -440,7 +441,8 @@ fn sigSize(comptime sig: Signature) usize {
     }
 
     // The size is the size of all arguments plus the padding after argument list
-    return argSize + 8;
+    // "(" + ")\n--\n\n" => 7
+    return argSize + 7;
 }
 
 fn sigArgs(comptime sig: Signature) ![]const []const u8 {

pydust/src/pytypes.zig

@@ -425,6 +425,10 @@ fn Doc(comptime definition: type, comptime name: [:0]const u8) type {
             if (@hasDecl(definition, "__doc__")) {
                 @memcpy(userDoc[docOffset..], definition.__doc__);
             }
+
+            // Add null terminator
+            userDoc[userDoc.len] = 0;
+
             break :blk userDoc;
         };
     };