serde dependency should be optional

See #16, release log:

• 5131174 by @yegor256: #16 serde is optional
• 07d4c05 by @yegor256: #19 desc
• 73196af by @yegor256: Merge pull request #58 from ye...
• 795ac8d by @yegor256: new benchmark results
• 07b4723 by @yegor256: #19 keyword
• e90d36c by @yegor256: #19 litemap
• cd73a14 by @yegor256: #19 a bit faster
• 22e26c3 by @yegor256: Merge pull request #56 from ye...
• 465ac76 by @yegor256: new benchmark results
• 7eb85f3 by @yegor256: #19 definitely faster
• 5281040 by @yegor256: Merge pull request #55 from ye...
• 059188f by @yegor256: new benchmark results
• f86e76d by @yegor256: #19 doc
• 542865c by @yegor256: Merge pull request #53 from ye...
• 6a659e5 by @yegor256: #19 10 in benchmark
• d728724 by @yegor256: new benchmark results
• 9b27fda by @yegor256: #19 license
• d92897c by @yegor256: #19 print date
• 45da6b1 by @yegor256: #19 doc
• 04410bf by @yegor256: Merge pull request #52 from ye...
• and 37 more...

Released by Rultor 1.74.7, see build log