[dannote]

Found while collapsing Figler's 52-function optional-attribute chain into one Enum pipeline.

• Generic std returns. mix rustq.std keeps a method's return type when it depends only on the receiver's own type parameters, and records those parameters in the order the type takes them. Typing binds them to the receiver's type arguments with the new Type.substitute/2. For example, name.as_ref() on an &Option<String> is an Option<&String>. The table grows from 173 entries to 293. A method whose return type uses its own parameters, like [T]::get's I::Output, is still left out.
• List literals whose items share a type are typed as a list of it.
• Enum.at on a stage fuses to .nth(index). Before, it was .collect().get(index).cloned() with an untyped collect.
• A filter over copied tuple items matches each item by reference, as |&(set, field)| set.
• Enum.count, length, and byte_size stay usize where one is expected, through a shared integer_count/2 helper. Before, they always cast to i64.
• The hand-written as_ref typing is removed, since the std table now covers it.

mix ci passes with 888 tests. With this branch, Figler's mix ci also passes, clippy included, apart from the dependency check that fails whenever RustQ comes from a local path.