A container for storing an 'a as a bin-io buffer. The advantages of storing a
value as an 'a t rather than an 'a are:
'a t is faster (it's just a blit) and it has the same bin-io
representation as 'a.However, every time you need to access 'a itself you need to call value, which
requires deserializing.
'a t is safe in the sense that you cannot construct an 'a t that doesn't store a
valid 'a. When deserializing an 'a, this requires actually constructing 'a. If
you'd like access to the 'a that's constructed during deserialization, see the
bin_reader_t_with_value below.
Deserializing requires actually constructing an 'a, so this reader gives you
access to that 'a, rather than just throwing it away. This is useful because you
often might want to do something like indexing 'a t by inspecting 'a at the time
that you read it.