Add a method for marshaling directly into a user-provided buffer.

Description

This is an implementation of the proposal in https://github.com/fxamacker/cbor/issues/520.

PR Was Proposed and Welcomed in Currently Open Issue

[ ] This PR was proposed and welcomed by maintainer(s) in issue #___
[x] Closes or Updates Issue #520

Checklist (for code PR only, ignore for docs PR)

[ ] Include unit tests that cover the new code
[x] Pass all unit tests
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As part of this, I moved the scratch byte array from a pooled buffer field to the stack as-needed.

                                                            │ master.txt  │   user-provided-buffer-inline.txt    │
                                                            │   sec/op    │   sec/op     vs base                 │
Marshal/Go_bool_to_CBOR_bool                                  45.33n ± 0%   45.41n ± 0%  +0.19% (p=0.033 n=10)
Marshal/Go_uint64_to_CBOR_positive_int                        63.97n ± 0%   64.06n ± 0%       ~ (p=0.209 n=10)
Marshal/Go_int64_to_CBOR_negative_int                         50.56n ± 1%   50.61n ± 1%       ~ (p=0.195 n=10)
Marshal/Go_float64_to_CBOR_float                              60.98n ± 0%   61.42n ± 0%  +0.72% (p=0.001 n=10)
Marshal/Go_[]uint8_to_CBOR_bytes                              77.63n ± 0%   74.29n ± 0%  -4.30% (p=0.000 n=10)
Marshal/Go_string_to_CBOR_text                                77.50n ± 1%   76.89n ± 0%  -0.79% (p=0.000 n=10)
Marshal/Go_[]int_to_CBOR_array                                286.9n ± 0%   288.9n ± 0%  +0.71% (p=0.000 n=10)
Marshal/Go_map[string]string_to_CBOR_map                      914.8n ± 1%   876.3n ± 1%  -4.20% (p=0.000 n=10)
Marshal/Go_map[string]interface{}_to_CBOR_map                 2.233µ ± 0%   2.260µ ± 0%  +1.21% (p=0.000 n=10)
Marshal/Go_struct_to_CBOR_map                                 1.391µ ± 1%   1.395µ ± 1%       ~ (p=0.839 n=10)
Marshal/Go_map[int]interface{}_to_CBOR_map                    2.155µ ± 0%   2.188µ ± 0%  +1.51% (p=0.000 n=10)
Marshal/Go_struct_keyasint_to_CBOR_map                        1.382µ ± 1%   1.371µ ± 1%  -0.80% (p=0.000 n=10)
Marshal/Go_[]interface{}_to_CBOR_map                          1.617µ ± 0%   1.671µ ± 0%  +3.37% (p=0.000 n=10)
Marshal/Go_struct_toarray_to_CBOR_array                       1.332µ ± 1%   1.329µ ± 1%       ~ (p=0.401 n=10)

I also tried updating the implementation of Marshal to delegated to MarshalToBuffer, which worked fine, but the benchmarks were marginally worse. I think this may just be the overhead of an extra function call being super visible in benchmark cases that are already very fast, or noise:

Marshal/Go_bool_to_CBOR_bool                                  45.33n ± 0%   46.59n ± 0%  +2.78% (p=0.000 n=10)
Marshal/Go_uint64_to_CBOR_positive_int                        63.97n ± 0%   64.98n ± 0%  +1.56% (p=0.000 n=10)
Marshal/Go_int64_to_CBOR_negative_int                         50.56n ± 1%   53.09n ± 1%  +4.98% (p=0.000 n=10)
Marshal/Go_float64_to_CBOR_float                              60.98n ± 0%   63.23n ± 0%  +3.69% (p=0.000 n=10)
Marshal/Go_[]uint8_to_CBOR_bytes                              77.63n ± 0%   74.98n ± 0%  -3.41% (p=0.000 n=10)
Marshal/Go_string_to_CBOR_text                                77.50n ± 1%   78.20n ± 0%  +0.89% (p=0.000 n=10)
Marshal/Go_[]int_to_CBOR_array                                286.9n ± 0%   286.9n ± 0%       ~ (p=0.364 n=10)
Marshal/Go_map[string]string_to_CBOR_map                      914.8n ± 1%   889.5n ± 1%  -2.77% (p=0.000 n=10)
Marshal/Go_map[string]interface{}_to_CBOR_map                 2.233µ ± 0%   2.204µ ± 1%  -1.30% (p=0.000 n=10)
Marshal/Go_struct_to_CBOR_map                                 1.391µ ± 1%   1.400µ ± 1%       ~ (p=0.108 n=10)
Marshal/Go_map[int]interface{}_to_CBOR_map                    2.155µ ± 0%   2.151µ ± 1%       ~ (p=0.342 n=10)
Marshal/Go_struct_keyasint_to_CBOR_map                        1.382µ ± 1%   1.336µ ± 0%  -3.33% (p=0.000 n=10)
Marshal/Go_[]interface{}_to_CBOR_map                          1.617µ ± 0%   1.601µ ± 0%  -1.02% (p=0.000 n=10)
Marshal/Go_struct_toarray_to_CBOR_array                       1.332µ ± 1%   1.343µ ± 1%  +0.79% (p=0.022 n=10)

I would still rather see Marshal call MarshalToBuffer so that they both benefit from all of the existing test coverage, unless the benchmark diff is a concern.

fxamacker / cbor