Bypass sorting overhead for single-entry maps.

[benluddy]

The map[int]int case is a single-entry map. This optimization is an easy win for a case that is not extraordinarily rare.

MarshalCanonical/Go_map[string]string_to_CBOR_map             895.9n ± 2%   899.3n ± 1%        ~ (p=0.912 n=10)
MarshalCanonical/Go_map[string]string_to_CBOR_map_canonical   1.763µ ± 4%   1.763µ ± 2%        ~ (p=0.753 n=10)
MarshalCanonical/Go_struct_to_CBOR_map                        317.0n ± 2%   311.1n ± 0%   -1.88% (p=0.000 n=10)
MarshalCanonical/Go_struct_to_CBOR_map_canonical              315.5n ± 1%   314.6n ± 4%        ~ (p=0.239 n=10)
MarshalCanonical/Go_map[int]int_to_CBOR_map                   194.8n ± 2%   196.2n ± 1%        ~ (p=0.839 n=10)
MarshalCanonical/Go_map[int]int_to_CBOR_map_canonical         326.3n ± 3%   190.9n ± 3%  -41.47% (p=0.000 n=10)
geomean                                                       464.5n        424.0n        -8.71%

Description

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue #___
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[fxamacker]

Nice! LGTM!