Improves unpack performance for vectors by allowing the compiler
to vectorize flatbuffers::Vector to std::vec::Vec conversions,
using the unstable trusted_len feature.
Internally, enables an optimization in src/alloc/vec/spec_extend.rs:
Previously, unpacking a flatbuffers::Vector called
SpecExtend::extend_desugared fallback, which inhibits vectorization
(due to a branch before every element move). Declaring TrustedLen
allows SpecExtend::extend_trusted, which LLVM can often vectorize
into a memcpy.
For [ubyte] vectors in particular, this turns a rather expensive
loop of 'mov BYTE PTR [rax+r13*1], bpl' into a `call memcpy`.
