Support a bigint type

[udoprog]

Support a arbitrary size, whole-number representation through a new core type called bigint.

This would translate to the following in various programming languages:

• Python - natively supported.
• JavaScript - https://github.com/MikeMcl/bignumber.js/
• Java - java.util.BigInteger
• C# - System.Numerics.BigInteger.
• Rust - https://github.com/rust-num/num-bigint

Mentoring Instructions

As a primer, it will help to read General instructions for how the compiler works.

This will mean adding a new keyword to the language, bigint. This is done in lib/lexer.

We will also need to have this new type in our AST, and RpIR.

Ast can be found in the lib/ast module, and RpIR is hosted in lib/core. Specifically the rp_type.rs file.

Finally we'll have to add support for the new type in each distinct backend. You can find an example of this in the Java backend here: https://github.com/reproto/reproto/blob/master/lib/backend-java/src/utils.rs#L41

The dynamic backends will probably require different handling.