Loadable extension

[gwenn]

• I started from scratch so there is should be no copyright issue unlike previous PRs
• how to handle va_list in sqlite3_api_routines fields ?(https://github.com/gwenn/rusqlite/blob/1308cdaa9dce731131bfdbfe18ee4bce9c7662a9/libsqlite3-sys/upgrade.sh#L29-L31) (https://www.gnu.org/software/libc/manual/html_node/Calling-Variadics.html)
• how to handle null pointer in sqlite3_api_routines fields ? (either omitted or disabled: see https://github.com/sqlite/sqlite/blob/4c40b7b819bed28a63a04eed32882ee46bc826fc/src/loadext.c#L135-L510)
• version mismatch may be too restrictive ? (see https://github.com/gwenn/rusqlite/blob/1308cdaa9dce731131bfdbfe18ee4bce9c7662a9/libsqlite3-sys/sqlite3/bindgen_bundled_version_ext.rs#L7497-L7502)
• minimal test(s) => see examples
• statically linked (see https://github.com/sqlite/sqlite/blob/4c40b7b819bed28a63a04eed32882ee46bc826fc/src/sqlite3ext.h#L702-L706) => which means that Connection::from_handle should be used directly instead of Connection::extension_init2 ?
• dylib seems wrong on mac (ok on linux): a cargo clean before cargo build fixes it

otool -L target/debug/examples/libloadable_extension.dylib
...
	/usr/lib/libsqlite3.dylib (compatibility version 9.0.0, current version 321.3.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1292.100.5)

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (d7ac6a1) 81.33% compared to head (e8c35fc) 81.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1362      +/-   ##
==========================================
- Coverage   81.33%   81.19%   -0.14%     
==========================================
  Files          49       50       +1     
  Lines       10229    10276      +47     
==========================================
+ Hits         8320     8344      +24     
- Misses       1909     1932      +23     

Files	Coverage Δ
libsqlite3-sys/src/error.rs	10.00% <ø> (ø)
src/error.rs	26.25% <ø> (ø)
src/lib.rs	84.19% <ø> (ø)
tests/deny_single_threaded_sqlite_config.rs	93.33% <100.00%> (+1.02%)	⬆️
src/inner_connection.rs	80.71% <0.00%> (-0.27%)	⬇️
src/trace.rs	99.06% <50.00%> (ø)
examples/load_extension.rs	0.00% <0.00%> (ø)
libsqlite3-sys/build.rs	64.74% <73.58%> (+0.14%)	⬆️

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[gwenn]

% cargo bloat --crates --example loadable_extension --features="loadable_extension modern_sqlite functions vtab trace"
...
24.3%  53.9% 378.5KiB libsqlite3_sys
16.2%  35.8% 251.6KiB std
 4.0%   8.8%  61.8KiB rusqlite
 0.3%   0.7%   5.0KiB bitflags
 0.2%   0.4%   3.1KiB atomic
 0.1%   0.1%     912B memchr
 0.0%   0.1%     496B [Unknown]
 0.0%   0.1%     368B loadable_extension
 0.0%   0.0%     320B hashbrown
 0.0%   0.0%     208B smallvec
 0.0%   0.0%     128B allocator_api2
45.2% 100.0% 702.3KiB .text section size, the file size is 1.5MiB

[gwenn]

@thomcc would you mind reviewing this PR ?

[gwenn]

@thomcc should I merge or wait ? (see #1412) Thanks.