Implement deref, derefmut and asref for currency

[ChrisSchinnerl]

That way Currency becomes nicer to use

Tests failing

[n8maninger]

note: an implementation of Div might be missing for currency::Currency
--> src/currency.rs:13:1
|
13 | pub struct Currency(u128);
| ^^^^^^^^^^^^^^^^^^^ must implement Div
note: the trait Div must be implemented
--> /rustc/7cf61ebde7b22796c69757901dd346d0fe70bd97/library/core/src/ops/arith.rs:449:1

error[E0369]: cannot subtract currency::Currency from currency::Currency
--> src/currency.rs:300:61
|
300 | ("9.999999999999999999999999 SC", Currency::siacoins(10) - Currency::new(1)),

[n8maninger]

Arithmetic is failing

[ChrisSchinnerl]

Arithmetic is failing

Damn and I felt so smart when cargo check was passing. Guess it doesn't build the tests.