generation of real benchmark functions for benchmarking v2 (#13224)

* function generation with _name working, need to modify signature

* WIP

* support custom BenchmarkResult<T> type

* full support for BenchmarkResult<T> on benchmark function defs

* support () return type for benchmark function defs that don't use ?

* uncomment

* fix where clause handling

* fix benchmark function call bodies

* proper parsing of return type

* add UI tests for bad return type

* fix detection of missing last_stmt with defined return type

* UI tests covering missing last_stmt

* properly detect and complain about empty benchmark function defs

* fix missing Comma in Result<T, BenchmarkError> parsing + test

* add additional UI test

* allow complex path for BenchmarkResult and BenchmarkError in fn defs

* add UI tests covering complex path for BenchmarkResult, BenchmarkError

* retain doc comments and attributes

* also add attributes to struct

* add docs for benchmark function definition support

* fix imports on benchmark example

* fix issue with unused variables in extrinsic call fn def

* fix up docs

* remove support for v2::BenchmarkResult because it was confusing

* fix typo

* remove ability to use custom T for Result<T, BenchmarkError> in v2

* use missing call error instead of empty_fn()

* remove unneeded match statement

* Add a proper QED

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

* fix other QED

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

* cargo fmt

* add an explicit error for non TypePath as return type

* tweak error warning and add a UI test for non TypePath return

* remove comment

* add docs about T and I generic params

* improve docs referring to section "below"

* pull out return type checking logic into its own function

* pull out params parsing into its own function

* pull out call_def parsing into its own function

* add doc comment for missing_call()

* replace spaces with tabs

* add a result-based example to the benchmarking examples

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Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>