Implement ink! chain extensions

[Robbepop]

Implements #585.

ToDo List

• Add unit tests for IR.
• Add a UI test for the whole feature + new syntax.
• Adjust ink_env definitions and remove crate feature.
• Add IR implementation.
• Add codegen implementation.
• Add proc. macro definitions.
• Create a design for handling and managing errors and error codes from chain extension methods.
• Implement #[ink::chain_extension] code generation for the new design.
• Write docs for usage and documenting details.
• Rename #[ink::contract] env_types parameter to env.
• Implement chain extension calling for the off-chain environment.
  • As the previous implementation the current off-chain environment implementation for calling and registering chain extension methods does not support access to the environment such as the runtime storage. This is a huge disadvantage and should be fixed as soon as possible in the rework of the off-chain environment.
• Find solution for using Self::ErrorCode in chain extension method signatures. Ideally users should be able to use it.
  • Actually users do not really need to make use of it since return type of Result<T, Self::ErrorCode> is implcit for expect_ok && !expect_output.
• Make chain extension codegen work for all configs:
  • expect_ok: false and expect_output: false
  • expect_ok: false and expect_output: true
  • expect_ok: true and expect_output: false
  • expect_ok: true and expect_output: true
• Think about better names for expect_output and expect_ok.
  • Found better design with handle_status: bool and returns_result: bool attributes that default to true.

[codecov-io]

Codecov Report

Merging #592 (5fcb008) into master (abd5cf1) will decrease coverage by 0.80%.
The diff coverage is 72.02%.

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
- Coverage   83.71%   82.90%   -0.81%     
==========================================
  Files         156      161       +5     
  Lines        6975     7353     +378     
==========================================
+ Hits         5839     6096     +257     
- Misses       1136     1257     +121     

Impacted Files	Coverage Δ
crates/env/src/api.rs	43.75% <ø> (ø)
crates/env/src/backend.rs	0.00% <ø> (ø)
crates/env/src/call/call_builder.rs	0.00% <0.00%> (ø)
crates/env/src/call/create_builder.rs	0.00% <0.00%> (ø)
crates/env/src/chain_extension.rs	0.00% <0.00%> (ø)
crates/env/src/engine/off_chain/impls.rs	52.63% <0.00%> (-5.34%)	⬇️
crates/env/src/engine/off_chain/mod.rs	85.71% <ø> (-3.90%)	⬇️
crates/env/src/engine/off_chain/test_api.rs	80.85% <ø> (ø)
crates/env/src/types.rs	44.44% <ø> (ø)
crates/lang/codegen/src/lib.rs	100.00% <ø> (ø)
... and 35 more

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[ascjones]

LGTM. As discussed the expect_ok and expect_output attributes are not perfect, but I can't think of better names for now and is certainly not a blocker to merge.

[Robbepop]

We concluded to try out how a new proposed syntax would look like instead of going with expect_output and expect_ok:

#[ink(
    extension = 1,
    handle_status = true|false,
    returns_result = true|false
)]
fn my_extension_method();

Where handle_status and returns_result both default to true.

[shawntabrizi]

nice :)