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_codes can be constructed explicitly from ADM_return_t values.

3. error_codes can be constructed explicitly from int32_t (for RPC types).

4. error_codes can be converted implicitly to ADM_return_t, which makes code less verbose when returning from C++ code to the C code.

5. error_codes are convertible to bool. This means that we can now do

       error_code ec = fun();
       if(ec) { /* there was an error, do something */ }

   whereas previously we were forced to explicitly check against ADM_SUCCESS.

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

6. error_codes 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:

      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 (closed)