Add safe wrappers to automatically free mbedtls types

[eddyashton]

To try and stop the general class of issues described in #738, we need to support 2 slightly different things:

1. Always call the relevant _free() if we've called _init(), for mbedtls structs. We could do this with a non-pointer auto-freeing wrapper struct.
2. Free things allocated in constructors.

The latter could be done with a pattern like try {} catch { X.reset(); }, but I think this approach is cleaner. Constructors populate local temporary structs, and only retain those (move-assigning them into members) at the end of the constructor, if it was successful. The wrapper type is a unique_ptr because it gives us the custom deleter hook and move-only semantics already, but we could write our own instead to remove the additional pointer indirection.

The wrapper is generated by a macro, producing a unique_ptr with a custom deleter but also a make_unique function that will call _init, for symmetry and safety. The interesting file is mbedtls_wrappers.h, the rest is just renames (mbedtls_foo => mbedtls::Foo, mbedtls_bar(&foo...) => mbedtls_bar(foo.get()...)).

[ghost]

mbedtls_safe_wrappers@16233 aka 20201203.11 vs master ewma over 50 builds from 15487 to 16228