Allow custom formatting

[mendelsshop]

This lets you use

#[trace(format_enter="a is {a} ...")
fn foo(a: u32) {
}

So for example, when you call foo with 1 it will print: Entering foo(a is 1 ...).
Currently, it is combined with prefix_enter or prefix_exit when printing.
You can also do custom formatting for when a function exits with format_exit.
For example, #[trace(format_enter="returning {r} ..."), r is used to denote
when to interpolate the return value, but it is easy to change to a different symbol.
This technically fixes #30, because you can do #[trace(format_exit=""),
and not interpolate anything.
Generally, the formatting follows the same rules and produces the same errors as format!,
but it does not (yet) allow for anything past basic interpolation, like pretty printing or padding.
Note: format_enter and or format_exit cannot be used with pretty.

[gsingh93]

I don't have time to do a thorough review, but looks good after a quick skim and the tests pass, so approved. Thanks!