Feature: All trace attribute arguments are named/keyword arguments

[taqtiqa-mark]

Currently the #[trace] attribute is a mixture of:

1. optional, required or prohibited arguments
2. named/keyword and positional arguments

This makes the attribute logic brittle and more difficult to extend than it needs to be. Specifically, the trace name is a positional argument which is both required and optional:

1. from the attribute logic PoV it is required
2. from the user PoV it is optional

Other arguments can be prohibited or required from the attribute logic PoV, e.g. enter_on_poll is prohibited for non-async functions, but required for async functions.

We can't do anything about the required/prohibited/optional properties - these arise naturally from the domain.

We can simplify required/prohibited/optional parsing logic by not complicating things with positional and named/keyword parameter categories.
That is, by removing the use of a positional arguments.

Specifically, in the course of issue #113, landing as PR #127, the following supported syntax has simplified matters by eliminating the need for a validation stage, leaving only a parse stage:

1. #[trace] fn f(){} shorthand for #[trace(name='f')] fn f(){}
2. #[trace(enter_on_poll=true)] async fn f(){} shorthand for #[trace(enter_on_poll=true,name='f')] async fn f(){} . Note: Here order is immaterial.
3. #[trace(name='a')] fn f(){}

The existing mixture of named and positional arguments would no longer be accepted:

1. #[trace] fn f(){} shorthand for #[trace('f')] fn f(){}
2. #[trace(enter_on_poll=true)] async fn f(){} shorthand for #[trace(f,enter_on_poll=true)] async fn f(){} . Note: Here order is material.
3. #[trace('a')] fn f(){}

This would improve the readability of code.
This would deviate from the tracing crate convention, depending on your PoV this is a positive or negative.

Once there is a comprehensive test suite in place and all bugs are eliminated - perhaps then we could consider adding subtle corner cases to the parsing logic by supporting a mixture of positional and named/keyword arguments?

Thoughts?

[andylokandy]

implemented in #153