only store tail needed in containers

[aep]

this sort of works in the call assign branch

new[map::key = char, +1000] m = map::make();
m.insert("hello", "world");

but its awkward, because its semantically char + tail.
it'll never work if we move to utf8

new[map::key = String, +1000] m = map::make();

will only store the String ref structure, not the actual content.

new[map::key = StringBuffer, +1000] m = map::make();

stores the entire stringbuffers tail, even if its half empty.

one solution to this is to enable dynamic tail sizes by using alloca instead of var decls for new calls,
something like

new str = string::from_cstr("foo");

could then do an strlen() to determine the tail needed is 3 bytes and alloca just that.

but i really hate it, because dynamically allocating stack has some real ugly security implications.
i can already see people copying unchecked strings from http connections and blowing up their stack

[jwerle]

hmm I really do like the idea of dynamic tail, but I do agree with the concerns