JS: performance of numeric primops

The JS implementation of some numeric primops use JavaScript's BigInt (see code in rts/js/arith.js).

For performance we should replace this with code only using JS number type.

See h$mul2Word32 for an example where we explicitly use numbers limited to 16-bit range to compute 32-bit multiplications. This was faster than allocating BigInts, performing a BigInt operation, and finally converting back to number values.

• plusWord64 !10873 (closed)
• minusWord64 !12125 (closed)
• quotWord64 !15720 (closed)
• remWord64 !15720 (closed)
• timesWord64 !12125 (closed)
• timesInt64 !12125 (closed)
• quotInt64 !15720 (closed)
• remInt64 !15720 (closed)
• plusInt64 !12125 (closed)
• minusInt64 !12125 (closed)
• quotWord32 !15720 (closed)
• remWord32 !15720 (closed)
• quotRemWord32 !15720 (closed)
• quotRem2Word32 !15720 (closed)