Avoid CallersFrames from escaping

[prashantv]

The runtime.CallersFrames method doesn't do much other than returning a struct pointer wrapping the passed in pcs. Calling this method twice ends up simplifying some of the logic (not having to format the first stack separately from the remaining), and ends up brining the performance of TakeStacktrace similar to master.

# benchstat comparing 3 runs of master vs this branch, each run using benchtime=10s
name            old time/op    new time/op    delta
TakeStacktrace    1.47µs ± 2%    1.48µs ± 2%   ~     (p=1.000 n=3+3)

name            old alloc/op   new alloc/op   delta
TakeStacktrace      496B ± 0%      496B ± 0%   ~     (all equal)

name            old allocs/op  new allocs/op  delta
TakeStacktrace      2.00 ± 0%      2.00 ± 0%   ~     (all equal)

[abhinav]

For the case #1052 optimizes, this is slightly slower but still better overall. Worth it versus regressing on the other cases.

name \ time/op            master.txt  1052.txt    1053.txt
AddCallerAndStacktrace-8  869ns ± 4%  670ns ± 2%  761ns ± 2%

name \ alloc/op           master.txt  1052.txt    1053.txt
AddCallerAndStacktrace-8   665B ± 0%   449B ± 0%   657B ± 0%

name \ allocs/op          master.txt  1052.txt    1053.txt
AddCallerAndStacktrace-8   5.00 ± 0%   3.00 ± 0%   4.00 ± 0%