stacktrace: fixes for goimports and inlining

[axw]

goimports formats differently depending on the
go version. Change how we initialise the radix
tree to fix this, while also reducing source
code size.

Update AppendCallerFrames to take a maximum number
of frames to append. Multiple frames may share a
PC, so it is not enough to just supply the callers
if we want to limit the output. If the supplied
count is negative, return all frames as before.

[codecov-io]

Codecov Report

Merging #127 into master will decrease coverage by 16.53%.
The diff coverage is 90%.

@@             Coverage Diff             @@
##           master     #127       +/-   ##
===========================================
- Coverage   75.54%   59.01%   -16.54%     
===========================================
  Files          60       75       +15     
  Lines        3108     4028      +920     
===========================================
+ Hits         2348     2377       +29     
- Misses        577     1467      +890     
- Partials      183      184        +1

Impacted Files	Coverage Δ
stacktrace/library.go	90% <100%> (+3.33%)	⬆️
error.go	73.2% <100%> (ø)	⬆️
stacktrace/stacktrace.go	77.61% <75%> (ø)	⬆️
utils_other.go	100% <0%> (ø)
transport/transporttest/recorder.go	0% <0%> (ø)
internal/pretty/pretty.go	0% <0%> (ø)
internal/apmdebug/debug.go	20% <0%> (ø)
internal/krtext/indent.go	0% <0%> (ø)
internal/krtext/wrap.go	0% <0%> (ø)
transport/transporttest/chan.go	0% <0%> (ø)
... and 8 more

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