Fix type inference of Bisection method

[devmotion]

Currently, the example in the README can't be inferred:

julia> using Roots

julia> f(x) = exp(x) - x^4;

julia> α₀,α₁,α₂ = -0.8155534188089607, 1.4296118247255556, 8.6131694564414;

julia> @code_warntype find_zero(f, (8,9), Bisection())
Variables
  #self#::Core.Const(Roots.find_zero)
  fs::Core.Const(f)
  x0::Tuple{Int64, Int64}
  method::Core.Const(Bisection())

Body::Any
1 ─ %1 = Roots.NullTracks()::Core.Const(Roots.NullTracks())
│   %2 = Core.NamedTuple()::Core.Const(NamedTuple())
│   %3 = Base.pairs(%2)::Core.Const(Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}())
│   %4 = Roots.:(var"#find_zero#23")(Roots.nothing, %1, false, %3, #self#, fs, x0, method)::Any
└──      return %4

The PR fixes this issue, also for brackets with mixed types (Base.extrema and hence also Roots._extrema are not type stable for tuples of different types).

[codecov]

Codecov Report

Merging #245 (7eeae0c) into master (a77396c) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   83.38%   83.46%   +0.08%     
==========================================
  Files          12       12              
  Lines        1950     1954       +4     
==========================================
+ Hits         1626     1631       +5     
+ Misses        324      323       -1     

Impacted Files	Coverage Δ
src/bracketing.jl	86.54% <100.00%> (ø)
src/find_zeros.jl	95.90% <100.00%> (ø)
src/utils.jl	73.25% <100.00%> (+2.52%)	⬆️
src/find_zero.jl	71.55% <0.00%> (ø)

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[jverzani]

Nice! Thanks again!