patch 8.1.1056: no eval function for Ruby

Problem:    No eval function for Ruby.
Solution:   Add rubyeval(). (Ozaki Kiichi, closes #4152)

runtime/doc/eval.txt

@@ -2521,6 +2521,7 @@ repeat({expr}, {count})		String	repeat {expr} {count} times
 resolve({filename})		String	get filename a shortcut points to
 reverse({list})			List	reverse {list} in-place
 round({expr})			Float	round off {expr}
+rubyeval({expr})		any	evaluate |Ruby| expression
 screenattr({row}, {col})	Number	attribute at screen position
 screenchar({row}, {col})	Number	character at screen position
 screencol()			Number	current cursor column
@@ -7432,6 +7433,17 @@ round({expr})							*round()*
 <			-5.0
 		{only available when compiled with the |+float| feature}
 
+rubyeval({expr})					*rubyeval()*
+		Evaluate Ruby expression {expr} and return its result
+		converted to Vim data structures.
+		Numbers, floats and strings are returned as they are (strings
+		are copied though).
+		Arrays are represented as Vim |List| type.
+		Hashes are represented as Vim |Dictionary| type.
+		Other objects are represented as strings resulted from their
+		"Object#to_s" method.
+		{only available when compiled with the |+ruby| feature}
+
 screenattr({row}, {col})					*screenattr()*
 		Like |screenchar()|, but return the attribute.  This is a rather
 		arbitrary number that can only be used to compare to the