Reexport bigdecimal (#341)

This commit pub-uses `bigdecimal` so that consumers can use the `bigdecimal`
type without have explicit exact-version dependency.

`bigdecimal` is a `<1.0` library, and so cargo will pull in additional
copies of it by default, as semver says each release of a `<1.0` crate is
backwards incompatible. So, if a consumer of `ion-rust` didn't have the
exact right version, then the `TryFrom<Decimal> for BigDecimal`
implementation wouldn't be accessible.

Fixes #302

Co-authored-by: Marc Bowes <[email protected]>

src/lib.rs

@@ -35,3 +35,10 @@ pub use reader::Reader;
 pub use symbol_table::SymbolTable;
 pub use system_event_handler::SystemEventHandler;
 pub use types::IonType;
+
+/// Re-exports of third party dependencies that are part of our public API.
+///
+/// See also: https://github.com/amzn/ion-rust/issues/302.
+pub mod external {
+    pub use bigdecimal;
+}

tests/reexport_external.rs

@@ -0,0 +1,17 @@
+use std::convert::TryInto;
+
+use ion_rs::external::bigdecimal::{BigDecimal, Zero};
+use ion_rs::types::decimal::Decimal;
+
+/// This test shows how the ion_rs integration with bigdecimal can be used
+/// through a reexport. This means that consumers of ion_rs can use this
+/// integration without having to specify the exact depdendency version.
+///
+/// See also: https://github.com/amzn/ion-rust/issues/302.
+#[test]
+fn bigdecimal_is_reexported() {
+    let ion_rs_type = Decimal::new(0, 0);
+    let integration_type: BigDecimal = ion_rs_type.try_into().expect("not negative zero");
+
+    assert_eq!((Zero::zero(), 0), integration_type.as_bigint_and_exponent())
+}