Move type checks up front for common cases

These cases are hit often in Rails apps (FFI due to Socket).

kernel/common/string.rb

@@ -121,7 +121,9 @@ def [](index, other = undefined)
     Rubinius.primitive :string_aref
 
     unless other.equal?(undefined)
-      if index.kind_of? Regexp
+      if index.kind_of?(Fixnum) && other.kind_of?(Fixnum)
+        return substring(index, other)
+      elsif index.kind_of? Regexp
         match, str = subpattern(index, other)
         Regexp.last_match = match
         return str

kernel/platform/struct.rb

@@ -258,6 +258,8 @@ def []=(field, val)
       raise "Unknown field #{field}" unless offset
 
       case type
+      when Fixnum
+        @pointer.set_at_offset(offset, type, val)
       when FFI::Type::Array
         if type.implementation == InlineCharArray
           (@pointer + offset).write_string StringValue(val), type.size
@@ -281,6 +283,8 @@ def [](field)
       case type
       when FFI::TYPE_CHARARR
         (@pointer + offset).read_string
+      when Fixnum
+        @pointer.get_at_offset(offset, type)
       when FFI::Type::Array
         type.implementation.new(type, @pointer + offset)
       when FFI::Type::StructByValue