Make multiexponents type stable

[jbshannon]

collect called on MultiExponents is type-unstable because MultiExponents has no defined eltype. This PR adds a method for Base.eltype() so that collect returns a Vector{Vector{Int64}} rather than a Vector{Any}.

Before:

julia> collect(multiexponents(3, 2))
6-element Array{Any,1}:
 [2, 0, 0]
 [1, 1, 0]
 [1, 0, 1]
 [0, 2, 0]
 [0, 1, 1]
 [0, 0, 2]

After:

julia> collect(multiexponents(3, 2))
6-element Vector{Vector{Int64}}:
 [2, 0, 0]
 [1, 1, 0]
 [1, 0, 1]
 [0, 2, 0]
 [0, 1, 1]
 [0, 0, 2]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.14%. Comparing base (ac70c46) to head (1497df6).
Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   96.97%   97.14%   +0.16%     
==========================================
  Files           7        8       +1     
  Lines         728      806      +78     
==========================================
+ Hits          706      783      +77     
- Misses         22       23       +1     

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[inkydragon]

LGTM