Create inline-lookup snippet

[miguelsousa]

Within a sub or pos statement, the lookup snippet shouldn't expand to a block.

feature test {
    sub [a e i o u] f' lookup CNTXT_LIGS i' n' lookup CNTXT_SUB;
    sub [a e i o u] c' lookup CNTXT_LIGS t' s' lookup CNTXT_SUB;
} test;

feature test {
    pos T' lookup CNTXT_PAIR_POS [o c]' @ALL_MARKS' lookup CNTXT_MARK_TO_BASE;
} test;