pass stmt.QueryString to QueryContext and ExecContext calls

[Songmu]

This is to allow pre-rewriting by hooks.

What I want to do with this is to embed comments in the publish query, just like Teng and DBIx::Sunny in Perl.

It may be necessary to handle Prepare as well, but I'm not doing so at present.

There may be a little overhead because the assignment to stmt is always made for each invocation. What do you think?

[codecov]

Codecov Report

Merging #61 (9abdebe) into main (cf38712) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   74.09%   74.03%   -0.06%     
==========================================
  Files          11       11              
  Lines         938      936       -2     
==========================================
- Hits          695      693       -2     
  Misses        174      174              
  Partials       69       69              

Impacted Files	Coverage Δ
conn.go	69.18% <100.00%> (-0.33%)	⬇️

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[coveralls]

Coverage decreased (-0.03%) to 78.538% when pulling 9abdebe on Songmu:overwrite-query into cf38712 on shogo82148:main.

[shogo82148]