Maybe change the representation of Registers in std::rt::context

[catamorphism]

See FIXMEs in contexts.rs (formerly XXXes):

// XXX: Registers is boxed so that it is 16-byte aligned, for storing
// SSE regs.  It would be marginally better not to do this. In C++ we
// use an attribute on a struct.
// XXX: It would be nice to define regs as `~Option<Registers>` since
// the registers are sometimes empty, but the discriminant would
// then misalign the regs again.

[toddaaro]

Would this be a performance gain to do correctly? If so this seems like an easy one and likely something we should do sooner than later.

[sanxiyn]

Triage. This moved to green::context, otherwise the same.

[thestinger]

#17325 means this is no longer relevant to the standard libraries