Merge branch 'amiranda/81-make-admire-error_code-a-fully-fledged-class' into 'main'

Resolve "Make `admire::error_code` a fully fledged class"

This MR redefines the `admire::error_code` typedef transforming it into
a fully-fledged C++ object.

This has the following advantages:

1. The default `error_code` constructor initializes to `ADM_SUCCESS`.
   Thus, we can now do `error_code ec;` instead of `error_code ec = ADM_SUCCESS;`.
2. `error_code`s can be constructed explicitly from `ADM_return_t` values.
3. `error_code`s can be constructed explicitly from `int32_t` (for RPC types).
4. `error_code`s can be converted implicitly to `ADM_return_t`, which makes code
   less verbose when returning from C++ code to the C code.
5. `error_code`s are convertible to `bool`. This means that we can now do

   ```c++
       error_code ec = fun();
       if(ec) { /* there was an error, do something */ }
   ```
   
   whereas previously we were forced to explicitly check against `ADM_SUCCESS`.

   ```c++
       error_code ec = fun();
       if(ec != ADM_SUCCESS) { /* there was an error, do something */ }
   ```

6. `error_code`s provide `value()`, `name()`, and `message()` functions that make
   them much more flexible.
7. Several `error_code` constants are defined to avoid using `ADM_return_t` 
   directly in C++ code, hopefully making the code more readable:
   
   ```c++
      constexpr error_code error_code::success = error_code{ADM_SUCCESS};
      constexpr error_code error_code::snafu = error_code{ADM_ESNAFU};
      constexpr error_code error_code::bad_args = error_code{ADM_EBADARGS};
      constexpr error_code error_code::out_of_memory = error_code{ADM_ENOMEM};
      constexpr error_code error_code::entity_exists = error_code{ADM_EEXISTS};
      constexpr error_code error_code::no_such_entity = error_code{ADM_ENOENT};
      constexpr error_code error_code::adhoc_in_use = error_code{ADM_EADHOC_BUSY};
      constexpr error_code error_code::other = error_code{ADM_EOTHER};
   ```

Also, most of the implementation is `constexpr`, which is always nice.

Closes #81

See merge request !61