Add show function to spans

[Paalon]

Depends on #183. Add a show function to Span for debugging.
Shown as the following format:

(line, column) ∈ (10, 20)...(20, 10)

[kescobo]

#183 now merged - resolve conflicts and we can merge this too.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.20%. Comparing base (3b2b353) to head (505b204).

Files	Patch %	Lines
src/span.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #222   +/-   ##
=======================================
  Coverage   87.19%   87.20%           
=======================================
  Files          15       15           
  Lines        1664     1665    +1     
=======================================
+ Hits         1451     1452    +1     
  Misses        213      213           

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[Paalon]

Now, conflicts are resolved.