Improve on Mapping::contains + Migrate examples to new API

[cmichi]

Follow-up to #1224.

[paritytech-cicd-pr]

🦑 📈 ink! Example Contracts ‒ Changes Report 📉 🦑

These are the results when building the examples/* contracts from this branch with cargo-contract 1.3.0-824c2f6 and comparing them to ink! master:

	Δ Optimized Size	Δ Used Gas	Total Optimized Size	Total Used Gas
accumulator			1.04 K
adder			2.13 K
contract-introspection			2.37 K
contract-terminate			0.94 K	275_000
contract-transfer			8.13 K	75_000
data-structures			1.73 K
delegate-calls			2.96 K	76_276
delegator			6.38 K	232_281
dns	+0.05 K		8.89 K	225_000
erc1155	-0.05 K		17.30 K	450_000
erc20			8.49 K	225_000
erc721	+0.01 K		11.82 K	600_000
flipper			1.31 K	75_000
forward-calls			2.88 K	151_427
incrementer			1.21 K
multisig	-0.03 K	-22	25.12 K	470_156
rand-extension			3.92 K	75_000
seal-code-hash			1.58 K
set-code-hash			1.57 K	150_000
subber			2.15 K
trait-erc20			8.77 K	225_000
trait-flipper			1.00 K	75_000
trait-incrementer			1.19 K	150_000
updated-incrementer			9.62 K
upgradeable-flipper			1.55 K

Link to the run | Last update: Fri May 13 12:03:38 CEST 2022

[agryaznov]

LGTM

[codecov-commenter]

Codecov Report

Merging #1242 (a27a53b) into master (961c7ef) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

❗ Current head a27a53b differs from pull request most recent head 1506bb3. Consider uploading reports for the commit 1506bb3 to get more accurate results

@@            Coverage Diff             @@
##           master    #1242      +/-   ##
==========================================
- Coverage   79.12%   79.05%   -0.08%     
==========================================
  Files         227      227              
  Lines        8638     8645       +7     
==========================================
- Hits         6835     6834       -1     
- Misses       1803     1811       +8     

Impacted Files	Coverage Δ
crates/engine/src/ext.rs	70.17% <0.00%> (-3.00%)	⬇️
crates/env/src/engine/off_chain/impls.rs	44.70% <0.00%> (ø)
crates/storage/src/lazy/mapping.rs	54.28% <ø> (ø)
crates/lang/ir/src/ir/attrs.rs	81.99% <0.00%> (-0.28%)	⬇️

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[HCastano]

From Michi, as to why this is marked as dont-merge

I'm just not merging it now because it breaks the examples/ if used with latest ink! 3.0.1 release.