Tag ink! crates with 2.0.0 release

[Robbepop]

This PR performs the setup needed to roll out the first ink! release.

[codecov-io]

Codecov Report

Merging #353 into master will increase coverage by 30.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           master      #353       +/-   ##
============================================
+ Coverage   69.88%   100.00%   +30.11%     
============================================
  Files          75         1       -74     
  Lines        6481         7     -6474     
============================================
- Hits         4529         7     -4522     
+ Misses       1952         0     -1952     

Impacted Files	Coverage Δ
lang/macro/tests/compile_tests.rs
core/src/env/engine/off_chain/db/console.rs
core/src/env/api.rs
core/src/env/engine/off_chain/db/accounts.rs
core/src/storage/collections/vec/tests.rs
core/src/storage/collections/hash_map/tests.rs
lang/macro/src/codegen/mod.rs
abi/src/lib.rs
core/src/env/engine/off_chain/mod.rs
lang/macro/src/extensions.rs
... and 66 more

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[athei]

LGTM. I mean it just bumps version numbers.