This commit changes SQLiteConn.Exec to use the raw Go query string
instead of repeatedly converting it to a C string (which it would do for
every statement in the provided query). This yields a ~20% performance
improvement for a query containing one statement and a significantly
larger improvement when the query contains multiple statements as is
common when importing a SQL dump (our benchmark shows a 5x improvement
for handling 1k SQL statements).

Additionally, this commit improves the performance of Exec by 2x or more
and makes number and size of allocations constant when there are no bind
parameters (the performance improvement scales with the number of SQL
statements in the query). This is achieved by having the entire query
processed in C code thus requiring only one CGO call.

The speedup for Exec'ing single statement queries means that wrapping
simple statements in a transaction is now twice as fast.

This commit also improves the test coverage of Exec, which previously
failed to test that Exec could process multiple statements like INSERT.
It also adds some Exec specific benchmarks that highlight both the
improvements here and the overhead of using a cancellable Context.

This commit is a slimmed down and improved version of PR mattn#1133:
  mattn#1133

```
goos: darwin
goarch: arm64
pkg: github.com/mattn/go-sqlite3
cpu: Apple M1 Max
                                       │    b.txt     │                n.txt                │
                                       │    sec/op    │   sec/op     vs base                │
Suite/BenchmarkExec/Params-10             1.434µ ± 1%   1.186µ ± 0%  -17.27% (p=0.000 n=10)
Suite/BenchmarkExec/NoParams-10          1267.5n ± 0%   759.2n ± 1%  -40.10% (p=0.000 n=10)
Suite/BenchmarkExecContext/Params-10      2.886µ ± 0%   2.517µ ± 0%  -12.80% (p=0.000 n=10)
Suite/BenchmarkExecContext/NoParams-10    2.605µ ± 1%   1.829µ ± 1%  -29.81% (p=0.000 n=10)
Suite/BenchmarkExecStep-10               1852.6µ ± 1%   582.3µ ± 0%  -68.57% (p=0.000 n=10)
Suite/BenchmarkExecContextStep-10        3053.3µ ± 3%   582.0µ ± 0%  -80.94% (p=0.000 n=10)
Suite/BenchmarkExecTx-10                  4.126µ ± 2%   2.200µ ± 1%  -46.67% (p=0.000 n=10)
geomean                                   16.40µ        8.455µ       -48.44%

                                       │      b.txt      │                n.txt                │
                                       │      B/op       │    B/op     vs base                 │
Suite/BenchmarkExec/Params-10                 248.0 ± 0%   240.0 ± 0%    -3.23% (p=0.000 n=10)
Suite/BenchmarkExec/NoParams-10              128.00 ± 0%   64.00 ± 0%   -50.00% (p=0.000 n=10)
Suite/BenchmarkExecContext/Params-10          408.0 ± 0%   400.0 ± 0%    -1.96% (p=0.000 n=10)
Suite/BenchmarkExecContext/NoParams-10        288.0 ± 0%   208.0 ± 0%   -27.78% (p=0.000 n=10)
Suite/BenchmarkExecStep-10               5406674.50 ± 0%   64.00 ± 0%  -100.00% (p=0.000 n=10)
Suite/BenchmarkExecContextStep-10         5566758.5 ± 0%   208.0 ± 0%  -100.00% (p=0.000 n=10)
Suite/BenchmarkExecTx-10                      712.0 ± 0%   520.0 ± 0%   -26.97% (p=0.000 n=10)
geomean                                     4.899Ki        189.7        -96.22%

                                       │     b.txt     │               n.txt                │
                                       │   allocs/op   │ allocs/op   vs base                │
Suite/BenchmarkExec/Params-10              10.000 ± 0%   9.000 ± 0%  -10.00% (p=0.000 n=10)
Suite/BenchmarkExec/NoParams-10             7.000 ± 0%   4.000 ± 0%  -42.86% (p=0.000 n=10)
Suite/BenchmarkExecContext/Params-10        12.00 ± 0%   11.00 ± 0%   -8.33% (p=0.000 n=10)
Suite/BenchmarkExecContext/NoParams-10      9.000 ± 0%   6.000 ± 0%  -33.33% (p=0.000 n=10)
Suite/BenchmarkExecStep-10               7000.000 ± 0%   4.000 ± 0%  -99.94% (p=0.000 n=10)
Suite/BenchmarkExecContextStep-10        9001.000 ± 0%   6.000 ± 0%  -99.93% (p=0.000 n=10)
Suite/BenchmarkExecTx-10                    27.00 ± 0%   18.00 ± 0%  -33.33% (p=0.000 n=10)
geomean                                     74.60        7.224       -90.32%
```