Deny overflowing (and lossy) integer type cast operations

[davidsemakula]

Summary

Closes #_ (N/A)

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependent on the specific version of ink or pallet-contracts?

Enables clippy lints for overflowing (and lossy) integer type cast operations (i.e. as conversions)

Description

#1190 "Disallow[ed] unchecked arithmetic", but from this comment, it looks like the actual intent was to "guarantee that implicit overflows never happen", if that's the case then like the -C overflow-checks for rustc, -Dclippy::arithmetic_side_effects is actually NOT sufficient.
This is because type cast operations (i.e. as conversions) can also cause implicit overflows/underflows (as well as other lossy conversions).

This PR enables additional clippy lints (see list below) to "guarantee that overflowing/underflowing and lossy integer operations never happen (in the local crate)".

• https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_truncation
• https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_wrap
• https://rust-lang.github.io/rust-clippy/master/index.html#cast_sign_loss

NOTE: We intentionally ignore safe/lossless lints (e.g. cast_lossless), and lints where either the source or target type is not an integer type (e.g. cast_precision_loss, char_lit_as_u8, as_conversions e.t.c).

References:
https://rust-lang.github.io/rfcs/0560-integer-overflow.html#updates-since-being-accepted
rust-lang/rfcs#1019
https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics
https://doc.rust-lang.org/reference/expressions/operator-expr.html#overflow

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[davidsemakula]

Seeing as #1190 didn't include any tests, I'm not sure if it makes sense to add tests for both (this and #1190), or if they just simply don't add much value - as we're just enabling clippy lints.