Burnable

[ozgunozerk]

Fixes #7

This PR implements Burnable extension for Fungible Token. And fixes some minor issues along the way:

• Simplify issue template
• change value to amount in the codebase for the coherence with Stellar ecosystem
• use &Env everywhere instead of Env to allow more flexibility

Note: I’m usually biased towards introducing 1-2 changes per PR for keeping the scope narrow. But since the [value to amount] and [Env to &Env] changes are quite straight-forward and minimal, I’ve thought the effort required to separate them into other PRs would be a waste of time.

Second note: this is based against fungible-token branch for now to ease the review process, and hence is a draft PR. Once Fungible Token PR merges, this should be rebased against main.

[brozorec]

great job 💪