Add support for pretty-printing Decimal numbers

[alamb]

In the context of apache/datafusion#107 from @joshuataylor

Describe the solution you'd like
It should be possible to pretty-print columns of DataType::Decimal type. Pretty printing is used for datafusion and other projects to make human readable versions of Arrow arrays.

To do so, we would add support here:

arrow-rs/arrow/src/util/display.rs

Lines 269 to 271 in cf0c7d2

	_ => Err(ArrowError::InvalidArgumentError(format!(
	"Pretty printing not implemented for {:?} type",
	column.data_type()

Right now, you get an error such as

ArrowError(InvalidArgumentError("Pretty printing not implemented for Decimal(9, 0) type"))

[mgill25]

Hi @alamb I'd like to take this issue up if that's ok.

Looks like the change should be simple enough - just add the Decimal pattern to the match expression. I have a silly question however: Is this being tested somewhere?

Seems like there is no test in display.rs, but there are tests in pretty.rs. Which I tried to run these tests, it seems cargo is skipping them, and yet it doesn't skip the tests in other neighboring modules.

Am I overthinking this a bit? Let me know how to figure this one out. :) Thanks!

Reason for tests: I'd like to see the change in action, so to speak.

[nevi-me]

@mgill25 the pretty tests are likely skipped because prettyprintis an optional feature

[alamb]

Thanks @mgill25 ! As @nevi-me says the pretty print feature is behind a feature flag (as it baloons the dependency stack with something that does not support for WASM builds, as I recall)

To answer your question, the pretty print feature is tested here in CI:
https://github.com/apache/arrow-rs/blob/master/.github/workflows/rust.yml#L114

To test it locally, you can run cargo like:

          cargo test --features=prettyprint