rollup merge of #19774: tomjakubowski/rustdoc-consts-statics

Build `clean::ConstantItem` values in the `inline` module and
pretty-print the AST for inlined const items.

Doc strings are still missing from inlined constants (see #19773).

Partially address #18156, #19722, #19185

Fix #15821

r? @alexcrichton

src/librustdoc/clean/mod.rs

@@ -1201,6 +1201,7 @@ pub enum TypeKind {
     TypeEnum,
     TypeFunction,
     TypeModule,
+    TypeConst,
     TypeStatic,
     TypeStruct,
     TypeTrait,
@@ -1841,7 +1842,7 @@ impl Clean<Item> for doctree::Static {
     }
 }
 
-#[deriving(Clone, Encodable, Decodable)]
+#[deriving(Clone, Encodable, Decodable, Show)]
 pub struct Constant {
     pub type_: Type,
     pub expr: String,