patch 8.1.2342: random number generator in Vim script is slow

Problem:    Random number generator in Vim script is slow.
Solution:   Add rand() and srand(). (Yasuhiro Matsumoto, closes #1277)

runtime/doc/eval.txt

@@ -2648,6 +2648,7 @@ pumvisible()			Number	whether popup menu is visible
 pyeval({expr})			any	evaluate |Python| expression
 py3eval({expr})			any	evaluate |python3| expression
 pyxeval({expr})			any	evaluate |python_x| expression
+rand([{expr}])			Number	get pseudo-random number
 range({expr} [, {max} [, {stride}]])
 				List	items from {expr} to {max}
 readdir({dir} [, {expr}])	List	file names in {dir} selected by {expr}
@@ -2761,6 +2762,7 @@ spellsuggest({word} [, {max} [, {capital}]])
 split({expr} [, {pat} [, {keepempty}]])
 				List	make |List| from {pat} separated {expr}
 sqrt({expr})			Float	square root of {expr}
+srand([{expr}])			List	get seed for |rand()|
 state([{what}])			String	current state of Vim
 str2float({expr})		Float	convert String to Float
 str2list({expr} [, {utf8}])	List	convert each character of {expr} to
@@ -7637,6 +7639,20 @@ range({expr} [, {max} [, {stride}]])				*range()*
 		Can also be used as a |method|: >
 			GetExpr()->range()
 <
+
+rand([{expr}])						*rand()*
+		Return a pseudo-random Number generated with an xorshift
+		algorithm using seed {expr}.  {expr} can be initialized by
+		|srand()| and will be updated by rand().
+		If {expr} is omitted, an internal seed value is used and
+		updated.
+
+		Examples: >
+			:echo rand()
+			:let seed = srand()
+			:echo rand(seed)
+			:echo rand(seed)
+<
 							*readdir()*
 readdir({directory} [, {expr}])
 		Return a list with file and directory names in {directory}.
@@ -9130,6 +9146,19 @@ sqrt({expr})						*sqrt()*
 		{only available when compiled with the |+float| feature}
 
 
+srand([{expr}])						*srand()*
+		Initialize seed used by |rand()|:
+		- If {expr} is not given, seed values are initialized by
+		  time(NULL) a.k.a. epoch time.
+		- If {expr} is given, return seed values which x element is
+		  {expr}.  This is useful for testing or when a predictable
+		  sequence is expected.
+
+		Examples: >
+			:let seed = srand()
+			:let seed = srand(userinput)
+			:echo rand(seed)
+
 state([{what}])						*state()*
 		Return a string which contains characters indicating the
 		current state.  Mostly useful in callbacks that want to do