This commit refactors the ValueGenerator class to be a template that can
work with any container type. Previously, one has to manually take care
of the used container by lazily iterating over it within a lambda. Now,
the `ValueGenerator` class itself takes care of all the iteration,
making it easier to use and less error-prone. The new base `Generator`
class is required to allow the `JsonEncoder` to handle generators in a
type-erased manner.
This commit intruduces a small helper class that wraps any writer and
provides a flush operation that performs the corresponding action if the
writer is an AsyncJsonWriter and does nothing otherwise.
Replacing invalid UTF-8 characters beforehand by our selves doesn't make
any sense, the serializer can literally perform the same replacement ops
with the exact same Unicode replacement character (U+FFFD) on its own.
So, why not just use it directly? Instead of wasting memory on a temporary
`String` object to always UTF-8 validate every and each value, we just
use the serializer to directly to dump the replaced char (if any) into
the output writer. No memory waste, no fuss!
