rework round and trunc

[jverzani]

This introduces these semantics for round and trunc

• both pass on non real values
• trunc and round return Sym objects (this is a change)
• round only uses the digits argument.

[codecov-commenter]

Codecov Report

Merging #473 (6dbebb0) into master (29c5bfd) will increase coverage by 0.58%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
+ Coverage   55.81%   56.39%   +0.58%     
==========================================
  Files          19       19              
  Lines        1179     1188       +9     
==========================================
+ Hits          658      670      +12     
+ Misses        521      518       -3     

Impacted Files	Coverage Δ
src/generic.jl	54.79% <80.00%> (+11.04%)	⬆️

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