Remove Default implementation for AccountId

[HCastano]

The problem here is that in the sr25519 and ed25519 curves the
zero-address public key has a known private key.

The Default implementation of AccountId returned the zero-address. Since developers
frequently reach for Default it is entirely possible that they could shoot themselves
in the foot unknowingly. For example, assigning a contract owner to the zero address
and not updating it, or using it as a burn address.

There's also an argument to be made that the concept of the Default address doesn't
make sense anyways, and we shouldn't be giving this specific address a special treatment.

Closes #1110.

[codecov-commenter]

Codecov Report

Merging #1255 (b7feae2) into master (3cec278) will decrease coverage by 5.66%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1255      +/-   ##
==========================================
- Coverage   70.38%   64.73%   -5.66%     
==========================================
  Files         207      207              
  Lines        6393     6381      -12     
==========================================
- Hits         4500     4131     -369     
- Misses       1893     2250     +357     

Impacted Files	Coverage Δ
crates/env/src/call/call_builder.rs	0.00% <0.00%> (ø)
crates/env/src/types.rs	0.00% <ø> (ø)
crates/primitives/src/types.rs	20.00% <ø> (ø)
crates/ink/ir/src/ir/contract.rs	0.00% <0.00%> (-91.67%)	⬇️
crates/ink/ir/src/ir/ink_test.rs	0.00% <0.00%> (-87.50%)	⬇️
crates/ink/ir/src/ir/trait_def/mod.rs	0.00% <0.00%> (-81.82%)	⬇️
crates/ink/ir/src/ir/storage_item/mod.rs	0.00% <0.00%> (-72.92%)	⬇️
crates/ink/ir/src/ir/storage_item/config.rs	0.00% <0.00%> (-68.75%)	⬇️
crates/ink/ir/src/ir/blake2.rs	20.00% <0.00%> (-60.00%)	⬇️
crates/ink/ir/src/ir/trait_def/config.rs	7.40% <0.00%> (-55.56%)	⬇️
... and 20 more

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