[env] pass salt argument to instantiate host function

[ascjones]

Fixes #593. Rel paritytech/substrate#7482

Allows specifying salt when instantiating a contract from within a contract e.g. (from delegator):

let adder = Adder::new(accumulator.clone())
    .endowment(total_balance / 4)
    .code_hash(adder_code_hash)
    .salt_bytes(salt)
    .instantiate()
    .expect("failed at instantiating the `Adder` contract");

todo

• How to choose type of salt - currently hardcoded to [u8; 4]. Is it possible/desirable to make this generic with the constructor builder. If not should Salt be added to the Environment trait?

[Robbepop]

doc tests seem to be failing

[codecov-io]

Codecov Report

Merging #599 (de5f0c6) into master (01f987d) will decrease coverage by 2.90%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #599      +/-   ##
==========================================
- Coverage   82.66%   79.76%   -2.91%     
==========================================
  Files         161      161              
  Lines        7414     7389      -25     
==========================================
- Hits         6129     5894     -235     
- Misses       1285     1495     +210     

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/create_builder.rs	0.00% <0.00%> (ø)
crates/env/src/engine/off_chain/impls.rs	52.63% <ø> (ø)
crates/lang/codegen/src/generator/cross_calling.rs	94.34% <ø> (ø)
crates/lang/src/env_access.rs	27.27% <ø> (ø)
crates/lang/ir/src/ir/contract.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/ir/src/ir/ink_test.rs	0.00% <0.00%> (-87.50%)	⬇️
crates/lang/ir/src/ir/item_mod.rs	59.06% <0.00%> (-32.22%)	⬇️
crates/lang/ir/src/ir/selector.rs	50.00% <0.00%> (-25.00%)	⬇️
... and 16 more

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[Robbepop]

LGTM