Properly mirror Wasmtime fuel API

[Robbepop]

• Remove Store::{add_fuel, consume_fuel}
• Remove Caller::{add_fuel, consume_fuel}
• Add Store::{get_fuel, set_fuel}
• Add Caller::{get_fuel, set_fuel}
• Add StoreContext::get_fuel
• Add StoreContextMut::{get_fuel, set_fuel}

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.14286% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 81.77%. Comparing base (18807fb) to head (39a4855). Report is 2 commits behind head on master.

Files	Patch %	Lines
crates/wasmi/src/store.rs	66.66%	6 Missing :warning:
crates/cli/src/main.rs	0.00%	4 Missing :warning:
crates/wasmi/src/func/caller.rs	0.00%	4 Missing :warning:
crates/cli/src/context.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1002 +/- ## ======================================= Coverage 81.76% 81.77% ======================================= Files 262 262 Lines 24327 24378 +51 ======================================= + Hits 19892 19936 +44 - Misses 4435 4442 +7 ```

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