A few miscellaneous fixes

[ssfrr]

• Fixes conv when eltype promotion is needed (one of the places I hit promotion between SampleBufs and Arrays fails #55 a lot)
• simplifies broadcasting code
• fixes issue where broadcasting ^ didn't work
• removes Compat and various old cruft

[codecov]

Codecov Report

Merging #57 into master will increase coverage by 2.69%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   71.12%   73.81%   +2.69%     
==========================================
  Files           6        6              
  Lines         374      359      -15     
==========================================
- Hits          266      265       -1     
+ Misses        108       94      -14

Impacted Files	Coverage Δ
src/SampledSignals.jl	100% <ø> (ø)	⬆️
src/SampleBuf.jl	80% <75%> (+9.83%)	⬆️
src/SampleStream.jl	88.5% <0%> (-0.64%)	⬇️

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