Use of `BCRYPTO_FORCE_BIGINT` may silently error and fallback to JS without BigInt

It may be expected that using `BCRYPTO_FORCE_BIGINT` would _always_ use `BigInt` or otherwise error. However this may not be the case, as it will fallback to use JS without `BigInt`, see: https://github.com/bcoin-org/bcrypto/blob/master/lib/bn.js

Start in lib/bn.js, where BCRYPTO_FORCE_BIGINT handling and the fallback behavior are located. Trace what happens when the setting is enabled without usable BigInt support; done means it consistently uses BigInt or reports an error instead of silently falling back to JavaScript without BigInt.

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