Delete unnecessary methods to fix invalidations

- Base.keys() already works on AbstractDict's so we don't need one for
  Accumulator.
- Base.eltype() is already defined for AbstractArray's and CircularBuffer is an
  AbstractVector so it doesn't need to define it again.
- A Base.convert() from AbstractArray to Array is already defined:
  https://github.com/JuliaLang/julia/blob/1ae41a2c0a3ba49d4b39dc4933dddf952b5f7f3c/base/array.jl#L612

Author: JamesWrigley

src/accumulator.jl

@@ -55,8 +55,6 @@ Base.setindex!(ct::Accumulator, x, v) = setindex!(ct.map, x, v)
 
 Base.haskey(ct::Accumulator, x) = haskey(ct.map, x)
 
-Base.keys(ct::Accumulator) = keys(ct.map)
-
 Base.values(ct::Accumulator) = values(ct.map)
 
 Base.sum(ct::Accumulator) = sum(values(ct.map))

src/circular_buffer.jl

@@ -154,17 +154,13 @@ Return the number of elements currently in the buffer.
 """
 Base.length(cb::CircularBuffer) = cb.length
 
-Base.eltype(::Type{CircularBuffer{T}}) where T = T
-
 """
     size(cb::CircularBuffer)
 
 Return a tuple with the size of the buffer.
 """
 Base.size(cb::CircularBuffer) = (length(cb),)
 
-Base.convert(::Type{Array}, cb::CircularBuffer{T}) where {T} = T[x for x in cb]
-
 """
     isempty(cb::CircularBuffer)