Move more parquet functionality behind experimental feature flag (#1032)

[tustvold]

Draft as builds on #1133

Which issue does this PR close?

Closes #1032

Rationale for this change

See ticket

What changes are included in this PR?

This hides a number of modules behind an "experimental" feature flag. These largely seem to contain implementation details, and so making them private will allow for faster iteration without having to worry about breaking clients that haven't explicitly opted in to instability.

Are there any user-facing changes?

Yes, this moves a number of APIs to be crate-local. I am skeptical people are using these APIs, but it is possible.

[tustvold]

This is necessary because without the experimental feature this will fail to compile complaining about compression being private.

FWIW this module is never documented now, but I opted to preserve the test.

You can see it running with:

$ cargo test --all-features --doc
    Finished test [unoptimized + debuginfo] target(s) in 0.04s
   Doc-tests parquet

running 19 tests
test src/arrow/array_reader.rs - arrow::array_reader::StructArrayReader::next_batch (line 1092) ... ignored
test src/arrow/array_reader.rs - arrow::array_reader::StructArrayReader::next_batch (line 1098) ... ignored
test src/arrow/array_reader.rs - arrow::array_reader::StructArrayReader::next_batch (line 1103) ... ignored
test src/file/mod.rs - file (line 64) - compile ... ok
test src/compression.rs - compression::CodecType (line 27) - compile ... ok
test src/file/mod.rs - file (line 29) - compile ... ok
test src/file/mod.rs - file (line 81) - compile ... ok
test src/record/api.rs - record::api::Row::get_column_iter (line 62) - compile ... ok
test src/column/mod.rs - column (line 38) - compile ... ok
test src/file/properties.rs - file::properties (line 22) ... ok
test src/schema/types.rs - schema::types::ColumnPath::append (line 674) ... ok
test src/arrow/mod.rs - arrow (line 58) ... ok
test src/schema/types.rs - schema::types::ColumnPath::string (line 663) ... ok
test src/schema/mod.rs - schema (line 22) ... ok
test src/schema/printer.rs - schema::printer (line 23) ... ok
test src/schema/parser.rs - schema::parser (line 24) ... ok
test src/file/statistics.rs - file::statistics (line 23) ... ok
test src/arrow/mod.rs - arrow (line 25) ... ok
test src/arrow/mod.rs - arrow (line 71) ... ok

test result: ok. 16 passed; 0 failed; 3 ignored; 0 measured; 0 filtered out; finished in 3.12s

[alamb]

Rebased on top of master

[tustvold]

Looking into test failures