⚡ Parse expected chars using String#getbyte

Using `string.getbyte(pos)` rather than `string[pos]` avoids the
allocation of a temporary string object.  This gives a speedup of ~5%
to most benchmarks.

Author: nevans

lib/net/imap/response_parser/parser_utils.rb

@@ -15,6 +15,7 @@ module Generator
 
           # we can skip lexer for single character matches, as a shortcut
           def def_char_matchers(name, char, token)
+            byte = char.ord
             match_name = name.match(/\A[A-Z]/) ? "#{name}!" : name
             char = char.dump
             class_eval <<~RUBY, __FILE__, __LINE__ + 1
@@ -27,15 +28,15 @@ def lookahead_#{name}?
 
               # use token or string peek
               def peek_#{name}?
-                @token ? @token.symbol == #{token} : @str[@pos] == #{char}
+                @token ? @token.symbol == #{token} : @str.getbyte(@pos) == #{byte}
               end
 
               # like accept(token_symbols); returns token or nil
               def #{name}?
                 if @token&.symbol == #{token}
                   #{SHIFT_TOKEN}
                   #{char}
-                elsif !@token && @str[@pos] == #{char}
+                elsif !@token && @str.getbyte(@pos) == #{byte}
                   @pos += 1
                   #{char}
                 end
@@ -46,7 +47,7 @@ def #{match_name}
                 if @token&.symbol == #{token}
                   #{SHIFT_TOKEN}
                   #{char}
-                elsif !@token && @str[@pos] == #{char}
+                elsif !@token && @str.getbyte(@pos) == #{byte}
                   @pos += 1
                   #{char}
                 else
@@ -109,7 +110,6 @@ def #{match_name}
                 end
               end
             RUBY
-
           end
 
         end