Rename enum variants to Rust-friendly names

[thomcc]

Fixes #1095.

I think this is technically breaking, because you can use an enum's variant to bring it into scope, but this is not allowed for constants. So code that does this would break:

use rusqlite::DbConfig::SQLITE_DBCONFIG_ENABLE_FKEY;
conn.db_config(SQLITE_DBCONFIG_ENABLE_FKEY)

Short of that, other cases should not be broken. EDIT: This was not possible for IndexConstraintOp, since it had the SQLITE_INDEX_CONSTRAINT_FUNCTION variant. I doubt many people use this, because we don't expose xFindFunction, so I just renamed it.

Other notes:

1. Cases which don't correspond to SQLite's constants (for example, UNKNOWN variants) are present as associated constants, but have had the capitalized versions deprecated, since the "compatibility with SQLite C API" argument doesn't hold. Some of these probably also should be TryFrom impls rather than From.

2. Some DbConfig values were gated on feature = "modern_sqlite". This has been removed, in favor of noting that they will fail if you try to use it in an unsupported version. This is useful if you want to toggle the feature if it's supported, but do nothing if not. For example, it seems reasonable to allow code like the following:

   if let Err(e) = conn.set_db_config(rusqlite::DbConfig::Defensive, true) {
       // Might be in an old version of SQLite, continue anyway...
       log::warn!("Failed to enable defensive mode: {:?}", e);
   }

3. I only bothered with the value tests for cases where there were a large number of variants.

4. In several cases, I also tried to improve the docs.

[codecov]

Codecov Report

Merging #1154 (d52300c) into master (9699b4a) will increase coverage by 0.22%.
The diff coverage is 82.25%.

@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
+ Coverage   78.22%   78.45%   +0.22%     
==========================================
  Files          47       47              
  Lines        5929     5926       -3     
==========================================
+ Hits         4638     4649      +11     
+ Misses       1291     1277      -14     

Impacted Files	Coverage Δ
src/session.rs	58.27% <16.66%> (+0.03%)	⬆️
src/hooks.rs	88.51% <25.00%> (ø)
src/config.rs	84.21% <66.66%> (-1.51%)	⬇️
src/vtab/mod.rs	55.39% <95.65%> (+4.20%)	⬆️
src/limits.rs	97.22% <95.83%> (-2.78%)	⬇️
src/vtab/array.rs	84.93% <100.00%> (ø)
src/vtab/series.rs	83.08% <100.00%> (ø)

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[gwenn]

You should have drop old entries.

[thomcc]

You should have drop old entries.

Hm, you mean I should delete the old variants?

If you feel that way, I'll deprecate and #[doc(hidden)] them. After a couple versions we can delete them. I don't want people to dread updating rusqlite, and deleting them all in one go would break very many users.