Use FixedVector for Class::m_[sp]initVec

Summary: This saves 32 bytes, bringing Class down to 280 in the lowptr build.
It's not quite a size class down yet, but it gets us that much closer.

Reviewed By: @jdelong

Differential Revision: D1609365
Shrink Class to 256 bytes

Summary: Shrink some fields that seem like they can be safely shrunk, apply
LowPtr, and move some stuff around.  Also reduce SProp from 56 to 48 bytes by
moving a field.

Reviewed By: @jdelong

Differential Revision: D1610488