This is supposed to be its own AO, but since it was only used in one
place, we inlined it. Now that it's also being used in the Temporal
proposal (Date.prototype.toTemporalInstant() specifically), it makes
sense to have it as a standalone function.
A small difference is that we now construct the SignedBigInteger without
casting to i32 but instead take the (known to be integral) double and
cast it to i64. Not perfect, but slightly better.
Also clean up the BigInt constructor a bit while we're here and sprinkle
some spec comments.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
