fix function display on v1.6

[aviatesk]

I found string(typeof(f).name) returns "typename(typeof(f))", which causes function display a bit weird (e.g. sin() on v1.6 (at least current master)

I believe using repr and string would make the logic mach simpler and robust in the future, so this PR.

I also improved a logic in a completion a bit

[codecov]

Codecov Report

Merging #339 into master will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   55.63%   55.54%   -0.10%     
==========================================
  Files          43       43              
  Lines        2644     2634      -10     
==========================================
- Hits         1471     1463       -8     
+ Misses       1173     1171       -2     

Flag	Coverage Δ
#unittests	55.54% <100.00%> (-0.10%)	⬇️

Impacted Files	Coverage Δ
src/completions.jl	78.77% <100.00%> (+0.39%)	⬆️
src/display/base.jl	37.09% <100.00%> (-3.82%)	⬇️

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