Change eltype methods for objects to types

[lgoettgens]

The docstring of eltype states

However the form that accepts a type argument should be defined for new types.

Chaning this should help julia infer types of iterator constructions like Iterators.reverse(...) for these types.

Furthermore, there were some methods installed that never triggered, e.g. eltype(::Type{SMat{T}}) where T does not get used as SMat has two type params. Changing this to eltype(::Type{<:SMat{T}}) where T fixes this

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.86%. Comparing base (a024dc6) to head (eb17d30).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Misc/Poly.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1634   +/-   ##
=======================================
  Coverage   75.85%   75.86%           
=======================================
  Files         361      361           
  Lines      113702   113701    -1     
=======================================
+ Hits        86249    86255    +6     
+ Misses      27453    27446    -7     

Files with missing lines	Coverage Δ
src/Misc/Cartesian.jl	73.33% <100.00%> (+1.33%)	⬆️
src/Misc/Integer.jl	63.94% <100.00%> (+0.15%)	⬆️
src/Misc/Primes.jl	85.00% <100.00%> (ø)
src/Sparse/Matrix.jl	71.46% <100.00%> (ø)
src/Sparse/Row.jl	74.39% <100.00%> (+0.73%)	⬆️
src/Misc/Poly.jl	69.42% <0.00%> (ø)

... and 24 files with indirect coverage changes