syntax: make OptVec immutable.

This is the first step to replacing OptVec with a new representation:
remove all mutability. Any mutations have to go via `Vec` and then make
to `OptVec`.

Many of the uses of OptVec are unnecessary now that Vec has no-alloc
emptiness (and have been converted to Vec): the only ones that really
need it are the AST and sty's (and so on) where there are a *lot* of
instances of them, and they're (mostly) immutable.
syntax: add the OwnedSlice vector wrapper.

This is a stand-in until we have a saner `~[T]` type (i.e. a proper
owned slice). It's a library version of what `~[T]` will be, i.e. an
owned pointer and a length.
Migrate all users of opt_vec to owned_slice.