patch 8.2.3438: cannot manipulate blobs

Problem:    Cannot manipulate blobs.
Solution:   Add blob2list() and list2blob(). (Yegappan Lakshmanan,
            closes #8868)

runtime/doc/usr_41.txt

@@ -723,6 +723,10 @@ Floating point computation:				*float-functions*
 	isinf()			check for infinity
 	isnan()			check for not a number
 
+Blob manipulation:					*blob-functions*
+	blob2list()		get a list of numbers from a blob
+	list2blob()		get a blob from a list of numbers
+
 Other computation:					*bitwise-function*
 	and()			bitwise AND
 	invert()		bitwise invert
@@ -1449,6 +1453,8 @@ is a List with arguments.
 Function references are most useful in combination with a Dictionary, as is
 explained in the next section.
 
+More information about defining your own functions here: |user-functions|.
+
 ==============================================================================
 *41.8*	Lists and Dictionaries