Fix SQL planner to support multibyte column names

[agatan]

Which issue does this PR close?

Closes #356

Rationale for this change

Please refer #356 for more detail.

What changes are included in this PR?

Slicing string will cause panic if the boundary is not correct as UTF-8.
To check if the first character is '@' or not, we should use s.chars().nth(0) instead of &s[0..1].

Are there any user-facing changes?

No.

[jorgecarleitao]

LGTM! 👍

[Dandandan]

😀 emoji support! 💯

[agatan]

Oh, sorry, I made a mistake in the test case.
I'll fix it now.

[alamb]

Looks great @agatan - thank you! I just started CI on this PR and once we get a clean run I think this one is ready to merge

[codecov-commenter]

Codecov Report

Merging #357 (be821d5) into master (6c050b8) will increase coverage by 0.12%.
The diff coverage is 97.50%.

❗ Current head be821d5 differs from pull request most recent head a5df543. Consider uploading reports for the commit a5df543 to get more accurate results

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
+ Coverage   75.72%   75.85%   +0.12%     
==========================================
  Files         143      143              
  Lines       23910    23814      -96     
==========================================
- Hits        18107    18064      -43     
+ Misses       5803     5750      -53     

Impacted Files	Coverage Δ
datafusion/src/physical_plan/functions.rs	92.77% <97.05%> (+3.19%)	⬆️
datafusion/src/sql/planner.rs	83.38% <100.00%> (+0.06%)	⬆️

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[alamb]

@agatan can you please fix the clippy error: https://github.com/apache/arrow-datafusion/pull/357/checks?check_run_id=2620378740

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: called `.nth(0)` on a `std::iter::Iterator`, when `.next()` is equivalent
   --> datafusion/src/sql/planner.rs:860:20
    |
860 |                 if id.value.chars().nth(0) == Some('@') {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^ help: try calling `.next()` instead of `.nth(0)`: `id.value.chars().next()`
    |
    = note: `-D clippy::iter-nth-zero` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth_zero

[agatan]

Thank you! I just fixed it now.