CI: test with Go 1.17 and 1.18

[abhinav]

We only support the two most recent minor releases.

[codecov]

Codecov Report

Merging #73 (eae9af6) into master (421b885) will increase coverage by 0.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   93.33%   94.01%   +0.68%     
==========================================
  Files           6        5       -1     
  Lines         120      117       -3     
==========================================
- Hits          112      110       -2     
+ Misses          5        4       -1     
  Partials        3        3              

Impacted Files	Coverage Δ
tracestack_old.go

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 421b885...eae9af6. Read the comment docs.