Since linearization_context is thread_local every time it is accessed
the compiler needs to emit code that checks if it was already
constructed and does so if it wasn't. Moreover, upon leaving the context
from the outermost scope the map needs to be cleared.
All these operations impose some performance overhead and aren't really
necessary if no buffers were linearised (the expected case). This patch
rearranges the code so that lineatization_context is trivially
constructible and the map is cleared only if it was modified.
It is needed for noexcept destruction, which we need for exception
safety in higher layers.
According to [1], erase() only throws if key comparison throws, and in
our case it doesn't.
[1] http://en.cppreference.com/w/cpp/container/unordered_map/erase
A large managed_bytes blob can be scattered in lsa memory. Usually this is
fine, but someone we want to examine it in place without copying it out, but
using contiguous iterators for efficiency.
For this use case, introduce with_linearized_managed_bytes(Func),
which runs a function in a "linearization context". Within the linearization
context, reads of managed_bytes object will see temporarily linearized copies
instead of scattered data.
