Remove erase_all function

[9names]

Remove unimplemented erase_all function.
erase_all is a default features of flash-algorithm so we need to add default-features = false in order to skip defining it.
Also renamed the rust-toolchain file to rust-toolchain.toml, since the version without file extension is the old style: "If both files are present in a directory, the latter is used for backwards compatibility"
Resolves #15

[thejpster]

Has this been tested with probe-rs? Can you paste a screenshot?

[9names]

Algo was copy-pasted from build.sh into RP2040.yaml, can't use target-gen yet.

[thejpster]

target-gen not working might be due to #14 (comment)

[9names]

It definitely is. I've tested it with the extra linker keep directives and it works.
The firmware size does grow substantially though, so we should keep it optional like in that PR.
I'm thinking of trying to fix that issue on the target-gen side as well (you don't need that info if updating an existing algo yaml, because it's already in the yaml)