make write_char defaultable

Author: bobzhang

builtin/builtin.mbti

@@ -803,6 +803,7 @@ pub(open) trait Logger {
   write_substring(Self, String, Int, Int) -> Unit
   write_char(Self, Char) -> Unit
 }
+impl Logger::write_char
 
 pub(open) trait Mod {
   op_mod(Self, Self) -> Self

builtin/traits.mbt

@@ -55,6 +55,11 @@ pub(open) trait Logger {
   write_char(Self, Char) -> Unit
 }
 
+///|
+impl Logger with write_char(self, value) {
+  self.write_string([value])
+}
+
 ///|
 /// Trait for types that can be converted to `String`
 pub(open) trait Show {