contract-sdk: use C abi instead of wasm

[ptrus]

The WASM abi seems quite broken, there are various problems with it:

• problems with u128 Broken WASM codegen with u128 and wasm_abi rust-lang/rust#88207
• problems with compiling multivalue functions https://github.com/ptrus/wasm-bug

Switch to using C abi. This requires modifying the exported functions to not use multivalue returns (which was only used for returning HostRegions currently).

[codecov]

Codecov Report

Merging #548 (22697b8) into main (9ad3543) will increase coverage by 0.09%.
The diff coverage is 83.60%.

@@            Coverage Diff             @@
##             main     #548      +/-   ##
==========================================
+ Coverage   71.94%   72.04%   +0.09%     
==========================================
  Files         101      101              
  Lines        7097     7150      +53     
==========================================
+ Hits         5106     5151      +45     
- Misses       1969     1977       +8     
  Partials       22       22              

Impacted Files	Coverage Δ
contract-sdk/specs/oas20/contract/src/lib.rs	97.34% <ø> (ø)
contract-sdk/src/abi/dispatch.rs	0.00% <ø> (ø)
contract-sdk/src/abi/env.rs	0.00% <ø> (ø)
contract-sdk/src/abi/storage.rs	0.00% <ø> (ø)
contract-sdk/src/memory.rs	0.00% <0.00%> (ø)
runtime-sdk/modules/contracts/src/abi/oasis/mod.rs	64.81% <60.00%> (+0.05%)	⬆️
...ime-sdk/modules/contracts/src/abi/oasis/storage.rs	60.82% <66.66%> (ø)
...time-sdk/modules/contracts/src/abi/oasis/memory.rs	90.12% <92.50%> (+2.31%)	⬆️
runtime-sdk/modules/contracts/src/abi/oasis/env.rs	61.90% <100.00%> (-0.60%)	⬇️
...untime-sdk/modules/contracts/src/abi/oasis/test.rs	100.00% <100.00%> (ø)

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