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

runtime/doc/if_ruby.txt

@@ -11,7 +11,8 @@ The Ruby Interface to Vim				*ruby* *Ruby*
 3. Vim::Buffer objects		|ruby-buffer|
 4. Vim::Window objects		|ruby-window|
 5. Global variables		|ruby-globals|
-6. Dynamic loading		|ruby-dynamic|
+6. rubyeval() Vim function	|ruby-rubyeval|
+7. Dynamic loading		|ruby-dynamic|
 
 {Vi does not have any of these commands}
 			*E266* *E267* *E268* *E269* *E270* *E271* *E272* *E273*
@@ -198,7 +199,16 @@ $curwin		The current window object.
 $curbuf		The current buffer object.
 
 ==============================================================================
-6. Dynamic loading					*ruby-dynamic*
+6. rubyeval() Vim function				*ruby-rubyeval*
+
+To facilitate bi-directional interface, you can use |rubyeval()| function to
+evaluate Ruby expressions and pass their values to Vim script.
+
+The Ruby value "true", "false" and "nil" are converted to v:true, v:false and
+v:null, respectively.
+
+==============================================================================
+7. Dynamic loading					*ruby-dynamic*
 
 On MS-Windows and Unix the Ruby library can be loaded dynamically.  The
 |:version| output then includes |+ruby/dyn|.