Add impl From for enum variant assist

Basically adds a From impl for tuple enum variants with one field. Added
to cover the fairly common case of implementing your own Error that can
be created from another one, although other use cases exist.

crates/ra_syntax/src/ast/make.rs

@@ -22,7 +22,8 @@ pub fn path_unqualified(segment: ast::PathSegment) -> ast::Path {
 pub fn path_qualified(qual: ast::Path, segment: ast::PathSegment) -> ast::Path {
     path_from_text(&format!("{}::{}", qual, segment))
 }
-fn path_from_text(text: &str) -> ast::Path {
+
+pub fn path_from_text(text: &str) -> ast::Path {
     ast_from_text(text)
 }