key_def: key_def.new() accept both 'field' and 'fieldno'

Closes #4519

@TarantoolBot document
Title: key_def.new() accept both 'field' and 'fieldno'

Before the patch key_def.new() took an index part
array as it is returned in <index_object>.parts: each
part should include 'type', 'fieldno', and what else
.parts element contains.

But it was not possible to create a key_def from an
index definition - the array passed to
<space_object>.create_index() 'parts' argument. Because
key_def.new() didn't recognize 'field' option. That
might be useful, when a key_def is needed on a remote
client, where a space object and its indexes do not
exist. And it would be strange to force a user to
create them just so as he would be able to access

    <net_box connection>.space.<space_name>.
        index.<index_name>.parts

As well as it would be crutchy to make a user manually
replace 'field' with 'fieldno' in its index definition
just to create a key_def.

Additionally, an ability to pass an index definition
to a key_def constructor makes the API more symmetric.

Note, that it still is not 100% symmetric, because a
user can't pass field names to the key_def
constructor. A space is needed for that anyway.

src/box/lua/key_def.c

@@ -88,8 +88,26 @@ luaT_key_def_set_part(struct lua_State *L, struct key_part_def *part,
 	lua_pushstring(L, "fieldno");
 	lua_gettable(L, -2);
 	if (lua_isnil(L, -1)) {
-		diag_set(IllegalParams, "fieldno must not be nil");
-		return -1;
+		lua_pop(L, 1);
+		/*
+		 * 'field' is an alias for fieldno to support the
+		 * same parts format as is used in
+		 * <space_object>.create_index() in Lua.
+		 */
+		lua_getfield(L, -1, "field");
+		if (lua_isnil(L, -1)) {
+			diag_set(IllegalParams,
+				 "fieldno or field must not be nil");
+			return -1;
+		}
+	} else {
+		lua_getfield(L, -2, "field");
+		if (! lua_isnil(L, -1)) {
+			diag_set(IllegalParams,
+				 "Conflicting options: fieldno and field");
+			return -1;
+		}
+		lua_pop(L, 1);
 	}
 	/*
 	 * Transform one-based Lua fieldno to zero-based

test/box-tap/key_def.test.lua

@@ -154,6 +154,29 @@ local key_def_new_cases = {
         }},
         exp_err = nil,
     },
+    --
+    -- gh-4519: key_def should allow the same options as
+    -- <space_object>.create_index(). That is, a field number
+    -- should be allowed to be specified as `field`, not only
+    -- `fieldno`.
+    --
+    {
+        'Success case; `field` is alias to `fieldno`',
+        parts = {{
+            field = 1,
+            type = 'unsigned'
+        }},
+        exp_err = nil,
+    },
+    {
+        'Field and fieldno can not be set both',
+        parts = {{
+            field = 1,
+            fieldno = 1,
+            type = 'unsigned'
+        }},
+        exp_err = 'Conflicting options: fieldno and field',
+    }
 }
 
 local test = tap.test('key_def')