ast/hir: Rename field-related structures

StructField -> FieldDef ("field definition")
Field -> ExprField ("expression field", not "field expression")
FieldPat -> PatField ("pattern field", not "field pattern")

Also rename visiting and other methods working on them.

compiler/rustc_ast/src/ast_like.rs

@@ -1,6 +1,6 @@
 use super::ptr::P;
 use super::tokenstream::LazyTokenStream;
-use super::{Arm, Field, FieldPat, GenericParam, Param, StructField, Variant};
+use super::{Arm, ExprField, FieldDef, GenericParam, Param, PatField, Variant};
 use super::{AssocItem, Expr, ForeignItem, Item, Local};
 use super::{AttrItem, AttrKind, Block, Pat, Path, Ty, Visibility};
 use super::{AttrVec, Attribute, Stmt, StmtKind};
@@ -187,8 +187,7 @@ derive_has_tokens_and_attrs! {
 // These ast nodes only support inert attributes, so they don't
 // store tokens (since nothing can observe them)
 derive_has_attrs_no_tokens! {
-    StructField, Arm,
-    Field, FieldPat, Variant, Param, GenericParam
+    FieldDef, Arm, ExprField, PatField, Variant, Param, GenericParam
 }
 
 // These AST nodes don't support attributes, but can