ARROW-4578: [JS] Ensure Float16 is zero-copy, add more native BigInt support

[trxcllnt]

This started as a continuation of #3634, but grew enough to deserve its own PR. I've made a PR to my own fork that highlights just the changes here: trxcllnt#8. I'll rebase this PR after #3634 is merged so only these changes are included.

This PR reverts the behavior of Float16Vector#toArray() back to returning a zero-copy slice of the underlying Uint16Array data, and exposes the copying behavior via new toFloat32Array() and toFloat64Array() methods. Float16Array.from() will also convert any incoming 32 or 64-bit floats to Uint16s if necessary.

It also adds tighter integration with the new BigInt, BigInt64Array, and BigUint64Array primitives (if available):

1. Use the native BigInt to convert/stringify i64s/u64s
2. Support the BigInt type in element comparator and indexOf()
3. Add zero-copy toBigInt64Array() and toBigUint64Array() methods to Int64Vector and Uint64Vector, respectively

0.4.0 added support for basic conversion to the native BigInt when available, but would only create positive BigInts, and was slower than necessary. This PR uses the native Arrays to create the BigInts, so we should see some speed ups there. Ex:

const vec = Int64Vector.from(new Int32Array([-1, 2147483647]))
const big = vec.get(0)
assert(big[0] === -1) // true
assert(big[1] === 2147483647) // true
const num = 0n + big // or BigInt(big)
assert(num === (2n ** 63n - 1n)) // true

JIRAs associated with this PR are:

• ARROW-4578 - Float16Vector toArray should be zero-copy
• ARROW-4579 - Add more interop with BigInt/BigInt64Array/BigUint64Array
• ARROW-4580 - Accept Iterables in IntVector/FloatVector from() signatures

[codecov-io]

Codecov Report

Merging #3653 into master will increase coverage by 2.85%.
The diff coverage is 88.73%.

@@            Coverage Diff             @@
##           master    #3653      +/-   ##
==========================================
+ Coverage   87.47%   90.33%   +2.85%     
==========================================
  Files         627       74     -553     
  Lines       80413     5413   -75000     
  Branches        0     1225    +1225     
==========================================
- Hits        70344     4890   -65454     
+ Misses       9960      515    -9445     
+ Partials      109        8     -101

Impacted Files	Coverage Δ
js/src/visitor/toarray.ts	98.21% <100%> (ø)
js/src/util/vector.ts	95.69% <100%> (ø)
js/src/util/bit.ts	100% <100%> (ø)
js/src/visitor/jsonvectorassembler.ts	100% <100%> (ø)
js/src/util/compat.ts	66.66% <60.86%> (ø)
js/src/util/bn.ts	72.22% <68.18%> (ø)
js/src/vector/float.ts	91.66% <86.66%> (ø)
js/src/vector/int.ts	90.9% <87.87%> (ø)
js/src/util/buffer.ts	93.7% <90%> (ø)
js/src/type.ts	93.72% <98.91%> (ø)
... and 701 more

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[domoritz]

This is great. Native BigInt support will get better and better and so having good support for it in Arrow is great.

[TheNeuralBit]

👍 I also appreciate you looking out for firefox users and handling the case where BigInt is not supported

Can you rebase now that #3634 is merged?

[trxcllnt]

@TheNeuralBit ok rebase is done. everything looks alright to me, let me know if you notice anything weird though.

[TheNeuralBit]

👍 LGTM