Refactor macro code

[coveralls]

Pull Request Test Coverage Report for Build 236

• 49 of 117 (41.88%) changed or added relevant lines in 14 files are covered.
• 5 unchanged lines in 4 files lost coverage.
• Overall coverage remained the same at 56.605%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
yaserde/src/de/mod.rs	0	1	0.0%
yaserde_derive/src/de/mod.rs	0	1	0.0%
yaserde_derive/src/ser/mod.rs	0	1	0.0%
yaserde/src/lib.rs	12	14	85.71%
yaserde_derive/src/common/attribute.rs	0	2	0.0%
yaserde_derive/src/ser/expand_struct.rs	0	7	0.0%
yaserde_derive/src/de/expand_enum.rs	0	11	0.0%
yaserde_derive/src/common/field.rs	0	16	0.0%
yaserde_derive/src/de/expand_struct.rs	0	27	0.0%
<!--	Total:	49	117	41.88%	-->

Files with Coverage Reduction	New Missed Lines	%
yaserde_derive/src/common/attribute.rs	1	70.29%
yaserde_derive/src/common/field.rs	1	0%
yaserde_derive/src/de/expand_enum.rs	1	0%
yaserde_derive/src/ser/expand_struct.rs	2	0%
<!--	Total:	5		-->

Totals
Change from base Build 227:	0.0%
Covered Lines:	917
Relevant Lines:	1620

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💛 - Coveralls

[jplatte]

ping @MarcAntoine-Arnaud