Replace namedValue with driver.NamedValue to avoid copying exec/query args

[charlievieth]

This commit replaces the namedValue type with driver.NamedValue this eliminates the need to create copy of the arguments slice in {SQLiteConn,SQLiteStmt}.{QueryContext,ExecContext}.

The driver.NamedValue type was added in 2016 (golang/go@707a833) so there shouldn't be any compatibility issues.

[rittneje]

LGTM. (driver.NamedValue was added in Go 1.8 specifically.) @mattn any concerns?

[mattn]

LGTM

[codecov-commenter]

Codecov Report

Base: 46.80% // Head: 46.59% // Decreases project coverage by -0.22% ⚠️

Coverage data is based on head (c1f721e) compared to base (1603038).
Patch coverage: 57.14% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
- Coverage   46.80%   46.59%   -0.22%     
==========================================
  Files          12       12              
  Lines        1534     1526       -8     
==========================================
- Hits          718      711       -7     
+ Misses        673      672       -1     
  Partials      143      143              

Impacted Files	Coverage Δ
sqlite3.go	52.64% <40.00%> (ø)
sqlite3_go18.go	77.77% <100.00%> (-4.58%)	⬇️

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