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]

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 ⚠️
crates/cli/src/main.rs	0.00%	4 Missing ⚠️
crates/wasmi/src/func/caller.rs	0.00%	4 Missing ⚠️
crates/cli/src/context.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           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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