implement fmt::Display instead of ToString

[vemoo]

As mentioned in the documentation for ToString implementing Display should be prefered.

This should be faster and use less memory because no intermediate Strings are created.

[coveralls]

Pull Request Test Coverage Report for Build 392

• 398 of 453 (87.86%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 92.549%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/ddl.rs	36	39	92.31%
src/ast/operator.rs	23	26	88.46%
src/ast/data_type.rs	21	31	67.74%
src/ast/mod.rs	160	179	89.39%
src/tokenizer.rs	21	41	51.22%

Totals
Change from base Build 386:	0.3%
Covered Lines:	4335
Relevant Lines:	4684

---

💛 - Coveralls

[benesch]

This looks fantastic! I've wanted to do this for a while.

I want to see if @nickolay has any thoughts before merging, though.

[nickolay]

if @nickolay has any thoughts

Nope - if you think this is right, let's land it!