besselj_zero is not type stable

[itsdfish]

Hi,

Thank you for putting this package together. While using besselj_zero in one of my projects, I noticed some type instability and high allocations. I'm not sure if there are any improvement which can be made.

Thanks!

@benchmark besselj_zero(0, 3)

BenchmarkTools.Trial: 10000 samples with 8 evaluations.
 Range (min … max):  3.780 μs …  1.405 ms  ┊ GC (min … max): 0.00% … 98.96%
 Time  (median):     4.108 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):   4.638 μs ± 14.065 μs  ┊ GC (mean ± σ):  3.00% ±  0.99%

  ▄█▇▆▆▅▄▂   ▁▁▁                                             ▂
  █████████▇█████▇▇▆▆▅▆▇██▆▇▆▅▅▅▆▇▇▇▅▆▅▅▅▅▆▅█▇▆▆▇▅▅▆▆▆▅▆▅▅▄▄ █
  3.78 μs      Histogram: log(frequency) by time     10.9 μs <

 Memory estimate: 1.84 KiB, allocs estimate: 45.

@code_warntype besselj_zero(0, 3)

MethodInstance for FunctionZeros.besselj_zero(::Int64, ::Int64)
  from besselj_zero(nu, n; order) @ FunctionZeros ~/.julia/packages/FunctionZeros/7kv6J/src/FunctionZeros.jl:52
Arguments
  #self#::Core.Const(FunctionZeros.besselj_zero)
  nu::Int64
  n::Int64
Body::Any
1 ─ %1 = FunctionZeros.:(var"#besselj_zero#1")(2, #self#, nu, n)::Any
└──      return %1

[jlapeyre]

Thanks for reporting!