Declare Failure early, in prep for fatalizing try.

We'll need it stubbed before encountering the first 'try' in the
setting.

src/core/Exception.pm

@@ -1,4 +1,3 @@
-my class Failure { ... }
 my role X::Comp { ... }
 my class X::ControlFlow { ... }
 

src/core/core_prologue.pm

@@ -6,6 +6,7 @@ my class Whatever { ... }
 my class HyperWhatever { ... }
 my class WhateverCode { ... }
 my class Cursor { ... }
+my class Failure { ... }
 
 # Stub these or we can't use any sigil other than $.
 my role Positional { ... }