With the API changes already done, notably .is_open(), the
implementation of the sqlite can and should now be private.
This also doesn't need to be #[repr(C)] anymore since the C API does
not need to access the fields in the struct.
Experiment with refactoring the internal sql interface a bit. My
original goal was to modify the schema and thus refactor to a state
where it would be sane to write tests for dc_sqlite_open() (and/or
however it ends up refactored) to assert schame changes before/after.
The C API allows passing a NULL pointer is for the callback function.
However when calling the callback nothing checks for this null pointer
and thus things fail badly. Even worse since the C API is defined
using an "fn pointer" rather than a "*-ptr" or raw pointer to the
function rust does not realise this can be invalid and therefore the
typechecker does not catch this even though there are no unsafe casts.
Fix this by making the callback an Option in rust, this can be easily
checked when calling. Also add a Context.call_cb() function which
simplifies calling the callback, hides the weird syntax due to the
function pointer and makes the call a little easier. Finally it also
means the option checking is only needed in one place.
For the C API this needs to check if this is a NULL pointer or not,
this is implicitly done by rust using the "nullable pointer
optimisation":
https://doc.rust-lang.org/nomicon/ffi.html#the-nullable-pointer-optimization
This stops using the deprecated libc::uint32_t and libc::uint64_t
types in favour of the native u32 and u64 types.
It also uses a newer nightly compiler to get rid of the incorrect
"unused no_mangle" warning when compiling incrementally.
Finally the newer compiler prefers us to be explicit when implementing
traits using the new dyn keyword.
This links the python bindings statically to libdeltachat.a if the
DCC_RS_DEV environment variable is set to the project's root. This is
a little simpler then requiring the manual CFLAGS and LD_LIBRARY_PATH
tweaking.
It also adds a script to easily invoke the integration tests locally
without forgetting steps.
