Add a check to TokenSymbol to match the same assertion as OCaml

[MartinMinkov]

Description

In OCaml, we have an assertion to check the length of a token symbol to be a max length of 6. We should additionally do this check-in SnarkyJS as well, to save the user time before sending it off to the network.

Presumably, SnarkyJS is already doing this check but the length seems to be off. If we pass in a very long token symbol, we get the following stack trace:

Error: prefix too long
    at prefixToField (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/hash.js:56:15)
    at Object.from (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/hash.js:141:21)
    at Object.set (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/account_update.js:379:86)
    at TokenContract.init (file:///workspace/deploy2berkeley/build/src/token-minaexplorer-repro.js:27:26)
    at file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/zkapp.js:89:113
    at runWith (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/global-context.js:55:18)
    at Function.runWith (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/global-context.js:14:37)
    at file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/zkapp.js:89:61
    at runWith (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/global-context.js:55:18)
    at Function.runWith (file:///workspace/deploy2berkeley/node_modules/snarkyjs/dist/node/lib/global-context.js:14:37)

We should just do the initial check of length 6 when setting the token symbol.

[garethtdavies]

Error is thrown here https://github.com/o1-labs/snarkyjs/blob/bcf3e8c2f8e90abfbf38a4848d1bf41cf173691f/src/lib/hash.ts#L84

Maybe something as simple as:

from(symbol: string): TokenSymbol {
    if (symbol.length > 6) throw Error('token symbol too long');
    let field = prefixToField(symbol);
    return { symbol, field };
  }

[mitschabaude]

The existing error is unrelated, it's if the string is too long to fit in one field element.
Yeah a simple error message like that would do it, preferably it should also mention the limit of 6