Fix building on ARM arch

[Fenex]

Hello.

bson-rs has a depending crate decimal which may using with optional feature "decimal128". Building decimal (last version 2.0.4) is terminating with error on ARM arch when I run

cargo build --features="decimal128"

The reason is the difference representation c_char type by two different platforms: c_char is i8 on x86\amd64 and is u8 on arm.

I guess you want ask me why I write here and don't create an issue inside 'decimal' repository. The fact is decimal crate already has a merged PR alkis/decimal#42 for fix that, however fixed version hasn't released. The trouble is, @alkis, author of the crate, has no active since April and I have no idea how much time we will not be able to get a fixed version on crates.io.

So, I suggest change source of the depending crate from crates.io to git commit with fixed compile error. It is the fastest and more safety path to resolve the problem.

On the other hand it is possible to migrate on another crate, for example, rust_decimal or other. It will be more complex and require some code changes for apply new library API.

[zonyitoo]

#125 I am hoping that in the future, we can switch to decimal128 crate.

[zonyitoo]

Using git path inside Cargo.toml will make this crate unpublishable to crates.io.

[saghm]

Unfortunately we can't merge this due to not being able to publish to crates.io with a raw git dependency, and the decimal crate has still not been updated, so closing this out. You can follow https://jira.mongodb.org/browse/RUST-36, which is the ticket we'll use to track work on the decimal BSON type.

[folliehiyuki]

The decimal crate got updated to 2.1.0 recently.
I think we can try again