similar for FieldVector converting to MVector/SizedVector

[mattsignorelli]

similar when called with a FieldVector should output an array type equal to the input type (with possibly different eltype if specified). However the current behavior does not do this. For immutable eltypes, it converts to an MVector:

julia> using StaticArrays

julia> mutable struct Quaternion{T <: Number} <: FieldVector{4, T}
         q0::T
         q1::T
         q2::T
         q3::T
       end

julia> q = Quaternion(1,2,3,4)
4-element Quaternion{Int64} with indices SOneTo(4):
 1
 2
 3
 4

julia> similar(q)  # This should be a `Quaternion{Int64}`
4-element MVector{4, Int64} with indices SOneTo(4):
 4472459824
          1
     563205
          1

And for mutable eltypes, a SizedVector:

julia> mutable struct MyNum{T} <: Number
       num::T
       end

julia> q = Quaternion{MyNum{Int64}}(MyNum(1),MyNum(2),MyNum(3),MyNum(4))
4-element Quaternion{MyNum{Int64}} with indices SOneTo(4):
 MyNum{Int64}(1)
 MyNum{Int64}(2)
 MyNum{Int64}(3)
 MyNum{Int64}(4)

julia> similar(q)
4-element SizedVector{4, MyNum{Int64}, Vector{MyNum{Int64}}} with indices SOneTo(4):
 #undef
 #undef
 #undef
 #undef