error: allow creating SpanTrace from Span directly (#1492)

## Motivation

I'm writing a tracing subscriber layer that captures tracing events to
display in an in-program UI in a somewhat structured manner; as such I
don't want to just use the fmt layer's format-to-a-single-string
approach, and instead capture (currently) the event metadata, fields,
timestamp, thread, and spantrace, to be displayed in a
filterable/searchable/interactable manner later.

Currently, the only way to capture a `SpanTrace` is via
`SpanTrace::capture`. This (rightfully) captures the `SpanTrace` of the
current execution. However, I'm currently inside `Subscribe::on_event`;
I have the span right here! Even if `Span::current` happens to be the
same during `on_event` as when the event was fired, it seems more
"proper" to create a `SpanTrace` from the span passed in to me, rather
than looking up `Span::current`.

## Solution

The actual code change is almost trivial: all that needs to happen is to
expose a `SpanTrace::new` that takes a `Span` in addition to the
existing `SpanTrace::current`.

tracing-error/src/backtrace.rs

@@ -69,6 +69,11 @@ pub struct SpanTrace {
 // === impl SpanTrace ===
 
 impl SpanTrace {
+    /// Create a new span trace with the given span as the innermost span.
+    pub fn new(span: Span) -> Self {
+        SpanTrace { span }
+    }
+
     /// Capture the current span trace.
     ///
     /// # Examples
@@ -96,9 +101,7 @@ impl SpanTrace {
     /// }
     /// ```
     pub fn capture() -> Self {
-        SpanTrace {
-            span: Span::current(),
-        }
+        SpanTrace::new(Span::current())
     }
 
     /// Apply a function to all captured spans in the trace until it returns