Fix typing issues in Nesterov and Broyden acceleration

[lostella]

Fix the following:

• Wrong method for iteration protocol is implemented on Nesterov sequences to get the eltype
• BroydenOperator is unnecessarily mutable, and typed in an overly complex and incomplete manner

Bump bugfix version number.

[codecov]

Codecov Report

Merging #73 (1de0e45) into master (9fe03c1) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   88.67%   88.93%   +0.26%     
==========================================
  Files          21       21              
  Lines         883      886       +3     
==========================================
+ Hits          783      788       +5     
+ Misses        100       98       -2     

Impacted Files	Coverage Δ
src/accel/broyden.jl	100.00% <100.00%> (ø)
src/accel/nesterov.jl	87.87% <100.00%> (+6.06%)	⬆️

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