Add support for rust_decimal and bigdecimal (#101)

schemars/src/json_schema_impls/decimal.rs

@@ -0,0 +1,31 @@
+use crate::gen::SchemaGenerator;
+use crate::schema::*;
+use crate::JsonSchema;
+
+macro_rules! decimal_impl {
+    ($type:ty) => {
+        decimal_impl!($type => Number, "Number");
+    };
+    ($type:ty => $instance_type:ident, $name:expr) => {
+        impl JsonSchema for $type {
+            no_ref_schema!();
+
+            fn schema_name() -> String {
+                $name.to_owned()
+            }
+
+            fn json_schema(_: &mut SchemaGenerator) -> Schema {
+                SchemaObject {
+                    instance_type: Some(InstanceType::$instance_type.into()),
+                    ..Default::default()
+                }
+                .into()
+            }
+        }
+    };
+}
+
+#[cfg(feature="rust_decimal")]
+decimal_impl!(rust_decimal::Decimal);
+#[cfg(feature="bigdecimal")]
+decimal_impl!(bigdecimal::BigDecimal);