Stabilize bindgen-at-build on more platforms

[0.3.5] (2025-28-04)

This release stabilizes the new "bindgen during build" approach of v0.3.4 to more platforms.

Support for newer libxml2 versions has been improved.

• CI support for v2.12.9, v2.13.8, v2.14.1

Thanks go to @wetneb, @charmitro and @TreyE for contributing.

Added

• cargo build: expose libxml2 version to main build script;
• creating a new Parser now initializes via bindings::xmlInitParser, called at most once.

Changes

• cargo build: mark max_align_t as opaque to fix i386 build failure
• cfg: adapt mutability of error pointer depending on libxml version
• change the return type of xmlGetNodeType from u32 to the more portable bindings::xmlElementType
• change the argument type of NodeType::from_int from u32 to the more portable bindings::xmlElementType
• protect Schema initialization to be closer to thread-safe (note that this wrapper is NOT thread-safe in general)

Removed

• The use of SchemaParserContext and SchemaValidationContext is currently NOT thread safe.
  Hence, The schema_test has been weakened to run in a single thread (future improvements welcome).