New rubinius tag format proposal

00 - ref
x1 - fixnum (only 1 bit used, leaving 31 bits for signed value)
10 - literal / symbol

bit patterns for important literals and values:
00000000 - non-literal value nil
00000010 - false
00000110 - true
00001010 - undef

xxxx1110 - symbols

which means the only two values with every value other than the secondmost least significant bit cleared are nil and false

#define RTEST(val) (val & 0xfffffffd)