xkbcomp/ast: combine expr_value_type into stmt_type This field is a funky attempt at type inference, or perhaps some optimization? Anyway, after careful examination I conclude it serves no purpose except specifying the type of a literal (string/integer/float/boolean/keyname) when `STMT_EXPR_VALUE` (i.e. literal). Remove it and replace `STMT_EXPR_VALUE` with specific statement types for each literal type. Signed-off-by: Ran Benita <ran@unusedvar.com>