Reduce allocations for execution despatch (#271)

[reisenberger]

The issue or feature being addressed

Delivers #271: Reduce heap allocations for execution despatch

Details on the issue fix or feature implementation

• Introduces struct-based execution despatch internally: reduces allocations per execution by 50% to 90% (50% to 90% saved). Full details will be posted on issue Execution-despatch optimization (reducing heap allocations) #271 and on v8.0.0 upgrade notes.
  • allied with use of readonly structs and in modifiers wherever possible to reduce struct copying.
• Maintains order-of-magnitude execution latency at <1microsecond (less than one-millionth-second) extra latency per policy for applying a Polly policy, for all policy types.
• Changes DelegateResult<TResult> to struct, as this is typically only used during an execution for short duration.
• Tidies TaskHelper (remnant of supporting older TFMs).
• Adds new execute overloads allowing passing objects in to delegates to be executed, without creating a closure:

void Execute<T1>(Action<Context, CancellationToken, T1> action, Context context, CancellationToken cancellationToken, T1 input1)
void Execute<T1, T2>(Action<Context, CancellationToken, T1, T2> action, Context context, CancellationToken cancellationToken, T1 input1, T2 input2)

TResult Execute<T1, TResult>(Func<Context, CancellationToken, T1, TResult> func, Context context, CancellationToken cancellationToken, T1 input1)
TResult Execute<T1, T2, TResult>(Func<Context, CancellationToken, T1, T2, TResult> func, Context context, CancellationToken cancellationToken, T1 input1, T2 input2)

Task ExecuteAsync<T1>(Func<Context, CancellationToken, bool, T1, Task> action, Context context, CancellationToken cancellationToken, bool continueOnCapturedContext, T1 input1)
Task ExecuteAsync<T1, T2>(Func<Context, CancellationToken, bool, T1, T2, Task> action, Context context, CancellationToken cancellationToken, bool continueOnCapturedContext, T1 input1, T2 input2)

Task<TResult> ExecuteAsync<T1, TResult>(Func<Context, CancellationToken, bool, T1, Task<TResult>> func, Context context, CancellationToken cancellationToken, bool continueOnCapturedContext, T1 input1)
Task<TResult> ExecuteAsync<T1, T2, TResult>(Func<Context, CancellationToken, bool, T1, T2, Task<TResult>> func, Context context, CancellationToken cancellationToken, bool continueOnCapturedContext, T1 input1, T2 input2)

(and similar overloads for .ExecuteAndCapture/Async(...))

Confirm the following

• I started this PR by branching from the head of the latest dev vX.Y branch, or I have rebased on the latest dev vX.Y branch, or I have merged the latest changes from the dev vX.Y branch
• I have targeted the PR to merge into the latest dev vX.Y branch as the base branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[grant-d]

Lovely stuff, great work