arrow

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

This started as a continuation of https://github.com/apache/arrow/pull/3634, but grew enough to deserve its own PR. I've made a PR to my own fork that highlights just the changes here: https://github.com/trxcllnt/arrow/pull/8. I'll rebase this PR after https://github.com/apache/arrow/pull/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:

```ts
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](https://issues.apache.org/jira/browse/ARROW-4578) - Float16Vector toArray should be zero-copy
* [ARROW-4579](https://issues.apache.org/jira/browse/ARROW-4579) - Add more interop with BigInt/BigInt64Array/BigUint64Array
* [ARROW-4580](https://issues.apache.org/jira/browse/ARROW-4580) - Accept Iterables in IntVector/FloatVector from() signatures

Author: ptaylor <paul.e.taylor@me.com>

Closes #3653 from trxcllnt/js/int-and-float-fixes and squashes the following commits:

69ee6f77 <ptaylor> cleanup after rebase
f44e97b3 <ptaylor> ensure truncated bitmap size isn't larger than it should be
7ac081ad <ptaylor> fix lint
6046e660 <ptaylor> remove more getters in favor of readonly direct property accesses
94d56334 <ptaylor> support BigInt in comparitor/indexOf
760a2199 <ptaylor> update BN to use BigIntArrays for signed/unsigned 64bit integers if possible
77fcd402 <ptaylor> add initial BigInt64Array and BigUint64Array support
d561204e <ptaylor> ensure Float16Vector.toArray() is zero-copy again, add toFloat32Array() and toFloat64Array() methods instead
854ae66f <ptaylor> ensure Int/FloatVector.from return signatures are as specific as possible, and accept Iterable<number>
4656ea55 <ptaylor> cleanup/rename Table + Schema + RecordBatch from -> new, cleanup argument extraction util fns
69abf406 <ptaylor> add initial RecordBatch.new and select tests
9c7ed3d4 <ptaylor> guard against out-of-bounds selections
a4222f81 <ptaylor> clean up: eliminate more getters in favor of read-only properties
8eabb1c0 <ptaylor> clean up/speed up: move common argument flattening methods into a utility file
b3b4f1fd <ptaylor> add Table and Schema assign() impls
79f9db1c <ptaylor> add selectAt() method to Table, Schema, and RecordBatch for selecting columns by index