Add Blake2s & XXHash utility functions (#66)

* Initial split of util & util-crypto

* Add initial XXHash version

* Blake2s wit tests & documentation

* Add sha512 (from NaCl)

* Add randomBytes (from NaCl)

* Flow fixes

* Update documentation

* Update latest @polkadot/dev

packages/util/docs/buffer.md

@@ -0,0 +1,97 @@
+# buffer
+
+Utility methods to convert to and from `Buffer` objects 
+
+- [bufferFromHex](#bufferfromhex) Creates a Buffer object from a hex string.
+- [bufferFromNumber](#bufferfromnumber) Creates a Buffer object from a number.
+- [bufferFromU8a](#bufferfromu8a) Creates a Buffer object from a Uint8Array.
+- [bufferToHex](#buffertohex) Creates a hex value from a Buffer object.
+- [bufferToNumber](#buffertonumber) Creates a Number value from a Buffer object.
+- [bufferToU8a](#buffertou8a) Creates a Uint8Array value from a Buffer object.
+
+## bufferFromHex
+
+Creates a Buffer object from a hex string. [(alias of hexToBuffer)](hex.md#hextobuffer)
+
+```js
+bufferFromHex (value?: string): Buffer
+```
+
+
+
+
+
+## bufferFromNumber
+
+Creates a Buffer object from a number. [(alias of numberToBuffer)](number.md#numbertobuffer)
+
+```js
+bufferFromNumber (value?: number): Buffer
+```
+
+
+
+
+
+## bufferFromU8a
+
+Creates a Buffer object from a Uint8Array. [(alias of u8aToBuffer)](u8a.md#u8atobuffer)
+
+```js
+bufferFromU8a (value?: string): Buffer
+```
+
+
+
+
+
+## bufferToHex
+
+Creates a hex value from a Buffer object. 
+
+```js
+bufferToHex (value?: Buffer): string
+```
+
+
+`null` inputs returns a `0x` result, `Buffer` values return the actual value as a `0x` prefixed hex value. Anything that is not a `Buffer` object throws an error.
+
+```js
+import { bufferToHex } from '@polkadot/util';
+
+bufferToHex(Buffer.from([1, 2, 3]); // => 0x010203
+```
+
+## bufferToNumber
+
+Creates a Number value from a Buffer object. 
+
+```js
+bufferToNumber (value?: Buffer): number
+```
+
+
+`null` inputs returns an NaN result, `Buffer` values return the actual value as a `Number`.
+
+```js
+import { bufferToNumber } from '@polkadot/util';
+
+bufferToNumber(Buffer.from([12, 34, 56])); // => 0x123456
+```
+
+## bufferToU8a
+
+Creates a Uint8Array value from a Buffer object. 
+
+```js
+bufferToU8a (value?: Buffer): string
+```
+
+
+`null` inputs returns an empty result, `Buffer` values return the actual value as a `Uint8Array`. Anything that is not a `Buffer` object throws an error.
+
+```js
+import { bufferToU8a } from '@polkadot/util';
+
+bufferToU8a(Buffer.from([1, 2, 3]));
+```