Equality is broken for non decimal types 

```(= (dc/decimal 123) :x)```
results in 
```
#object[Error Error: [DecimalError] Invalid argument: :x]
```

Due to the IEquiv implementation here:
https://github.com/funcool/decimal/blob/c0bee2ee62c62a3ba2a80008ec02e01daa0a736e/src/decimal/core.cljs#L752

It should check that the thing being compared is a number or decimal type. 

I will send a PR soon.