Refactor settings setters; use snapshots for events

[pscott]

1. Removes all set* functions
2. Introduces the update_settings function, similarly to sx-evm.
3. Updates events to use @ (snapshot) rather than values.

Closes #427

[pscott]

formatting failed but running cairo-format **/**.cairo doesn't work because I'm guessing some things have not been updated with v2... have to investigate :p

[Orland0x]

why not use NoUpdateArray::no_update()?

[pscott]

Yeah it was a miss; it's because at first I wanted to have something visually different to remind me that NoUpdateString::no_update() has not been implemented yet...

We can go with empty for now but I think we'll need to change in the future because "" is a valid metadata_uri and I'd be happier with something like No Update as a string to indicate it

[Orland0x]

so we can change to NoUpdateArray::no_update() ?

[pscott]

aye! will do

[Orland0x]

formatting failed but running cairo-format **/**.cairo doesn't work because I'm guessing some things have not been updated with v2... have to investigate :p

might be that cairo-format is different to scarb fmt we use the latter in the CI. i used scarb fmt and it worked