Add MSRV 1.70.0

[adamspofford-dfinity]

This adds a MSRV field to the library manifests, and a rust-toolchain.toml to ensure it is checked in CI.

I also took the opportunity to correct the clippy command in CI - clippy::all is a particular set of clippy lints, so -Dclippy::all won't flag rustc lints or manually #[warn]ed clippy lints.

[github-actions]

Benchmark for 931cfbc

Click to view benchmark

Test	Base	PR	%
Blob/&str	640.9±100.56µs	641.6±96.71µs	+0.11%
Blob/ByteBuf	355.3±90.32µs	353.3±85.99µs	-0.56%
Blob/Bytes	406.6±76.33µs	409.5±77.25µs	+0.71%
Blob/String	653.8±258.32µs	661.5±256.73µs	+1.18%
Collections/vec (text, nat)	47.7±0.82ms	47.5±0.72ms	-0.42%
Collections/vec int	24.2±0.11ms	23.8±0.09ms	-1.65%
Collections/vec int64	15.5±0.05ms	14.2±0.07ms	-8.39%
Collections/vec nat8	11.7±0.03ms	11.6±0.03ms	-0.85%
option list/1024	976.5±80.66µs	945.8±11.60µs	-3.14%
profiles/1024	1753.8±15.73µs	1767.8±53.61µs	+0.80%
variant list/1024	823.5±6.59µs	827.4±29.99µs	+0.47%