Refactor and modernize dispatch codegen (#933)

* implement Display for ink_lang::Error

* re-export HexLiteral trait from ink_lang_ir crate

* use big-letters for hex-encoded message and constructor identifiers in codegen

* improve panic messages in dispatch codegen for call and deploy fns

* improve macro hygiene in dispatch codegen

* add ir::Selector::into_be_u32 method

* improve and modernize dispatch selector namespace codegen
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