Move character literal processing to the lexer

To match the processing of integer literals, character literals are passed
as a string from lexer to parser then interpreted there.  This is just as
awkward as it was for integer literals, without the excuse that we used to
need the information about the dts version to process them correctly.

So, move character literal processing back to the lexer as well, cleaning
things up.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>