Improved performance of block parsing

[braydonf]

• Increases performance of block parsing by ~9x.
• Removes expensive precondition from readReverse
• Optimizes readUInt64LEBN
• Adds benchmarks for both of the above methods
• Adds additional tests around readUInt64LEBN

[pnagurny]

LGTM

[maraoz]

Can't this lose precision? See:

> 0x100000000 * 0x10000000 
1152921504606847000
> 0x100000000 * 0x10000000 + 1
1152921504606847000

Both answers give the same value. Does this mean that if second is 0 or 1 and first is 0x10000000, this would return the same value?

[braydonf]

It can lose precision with numbers equal or above 2 ^ 53.

> 0x1fffffffffffff === Math.pow(2, 53) - 1
// true

> 1152921504606847000 < 0x1fffffffffffff
// false

And in that case the number will be read directly into BN and not a JavaScript Number first: https://github.com/bitpay/bitcore/pull/1279/files#diff-17000cc0d07e4a49ab9cbafe33ca7231R101

[maraoz]

Ahhh, gotcha. I didn't understand the < 0x1fffffffffffff part. LGTM

[braydonf]

Rebased on master and added comment to code with info about number/array BN instantiation.