Improve conversion to Real

[jbransen]

While trying to work with Real values I encountered a couple of issues that this PR fixes:

• format!("{:?}", self.abs()) might use scientific notation, e.g. 1e-2, which is not accepted by at least Z3. Instead, I fixed the precision to 10 digits, which is somewhat arbitrary, but at least it always leads to valid terms, and is likely enough for most applications.
• Most conversions used Into instead of IntoWithStorage
• There was no Decimal support, while those can be nicely formatted exactly. I did put this behind a feature flag because it induces an extra dependency
• Division was using integer division instead of real division. Fixes Support Real Division #10