findPopAfter

auto source = `xyz`;
auto haystack = source;
const needle = `y`;
auto pop = haystack.findPopAfter(needle);
assert(pop == `xy`);
assert(haystack == `z`);

auto source = `xy`;
auto haystack = source;
const needle = `z`;
auto pop = haystack.findPopAfter(needle);
assert(pop is null);
assert(!pop);
assert(haystack == source);