BigInt-related API fixes and non-breaking changes

[mildsunrise]

BigInt-related API fixes and (for now, only) non-breaking changes.

• input conversions for BigInt:

  • all integer types (8, 16, 32, 64) now accept bigint and number

  • 64-bit integer types ([u]long, [u]int64, gtype) now convert the value to BigInt and then call [U]Int64Value()

  (the other integer types still convert to Number first, which means IIRC that if an out-of-range BigInt is passed, they'll get converted to trash rather than the low bits, as the user would expect. but it's better for performance and user can just & 0xFFFFFFFFn)

• GType consistency (these are breaking, but very little used so I think a fix like this is justified):

  • GValueToV8 now converts GType to BigInt to be consistent
  • GTypes only support conversion from BigInt now

• I've also annotated the places that should be changed to completely support BigInt, but would be a breaking change.

• other non-BigInt related fixes:

  • add missing entries for [u]int64 in CanConvertV8ToGValue

[romgrk]

You can include the breaking changes here directly, I'll release 1.0 right after it's merged.

[mildsunrise]

hmm I would wait a bit before the breaking release, just in case there are more breaking changes we haven't discovered yet? either way, +1

[romgrk]

The breaking changes need to land on master anyway, even if we wait before the release. You can add them here directly, it will be simpler.

[mildsunrise]

Perfect, done. This seems to be working but I'll test it some more to make sure.

[mildsunrise]

(note to self: parts of this PR have been merged as #301 and #322... I should take those out and rebase)