Keep track of attribute namespaces

[ctrlcctrlv]

Closes #13.

[ctrlcctrlv]

The changes just pushed (https://github.com/eminence/xmltree-rs/pull/33/commits/9216575598ba7bbed7b5c733dbac1c803130c327 – https://github.com/eminence/xmltree-rs/pull/33/commits/89729309f17334df2acb2d9e28f89ec8957e6501) improve code quality a tad, but don't change function or test results.

[fred@🍇葡萄🍇xmltree-rs]$ cargo test
   Compiling xmltree v0.11.0 (/home/fred/Workspace/xmltree-rs)
    Finished test [unoptimized + debuginfo] target(s) in 0.71s
     Running unittests (target/debug/deps/xmltree-c3a1a640866251c3)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/namespaces.rs (target/debug/deps/namespaces-7ccaa7bc5ab4ba2e)

running 5 tests
test test_default_ns ... ok
test test_empty_ns - should panic ... ok
test test_issue13 ... ok
test test_ns ... ok
test test_ns_rw ... ok

test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/suite.rs (target/debug/deps/suite-66ee467e004ba98b)

running 17 tests
test test_decl ... ok
test test_mal_02 ... ok
test test_new ... ok
test test_mal_01 ... ok
test test_nodecl ... ok
test test_mut ... ok
test test_mal_03 ... ok
test test_no_root_node ... ok
test test_04 ... ok
test test_03 ... ok
test test_text ... ok
test test_take ... ok
test test_rw ... ok
test test_parse_all ... ok
test test_02 ... ok
test test_write_with_config ... ok
test test_01 ... ok

test result: ok. 17 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests xmltree

running 1 test
test src/lib.rs - (line 7) - compile ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s

[ctrlcctrlv]

@eminence Don't merge until reading discussion above about two attributes w/same name from different namespaces. I didn't consider this case and more radical change is needed to handle it but since you were occupied with other matters didn't try to architect something as it's gonna have to be semver breaking.

[eminence]

Thanks for the heads up