Use ismutabletype on 1.7

[marius311]

Due to JuliaLang/julia#39037, on 1.7 we have:

julia> foo(;kwargs...) = (;kwargs...,).x^2
foo (generic function with 1 method)

julia> gradient(x->foo(x=x), 1)
ERROR: type DataType has no field mutable
Stacktrace:
  [1] getproperty
    @ ./Base.jl:37 [inlined]
  [2] adjoint
    @ ~/.julia/packages/Zygote/i1R8y/src/lib/lib.jl:287 [inlined]
  [3] _pullback
    @ ~/.julia/packages/ZygoteRules/OjfTt/src/adjoint.jl:57 [inlined]
  [4] _pullback
    @ ./boot.jl:605 [inlined]
  [5] _pullback(ctx::Zygote.Context, f::Type{NamedTuple{(:x,), Tuple{Int64}}}, args::Tuple{Int64})
    @ Zygote ~/.julia/packages/Zygote/i1R8y/src/compiler/interface2.jl:0
  [6] _pullback
    @ ./boot.jl:601 [inlined]
  [7] _pullback(ctx::Zygote.Context, f::Type{NamedTuple{(:x,)}}, args::Tuple{Int64})
    @ Zygote ~/.julia/packages/Zygote/i1R8y/src/compiler/interface2.jl:0
  [8] _pullback
    @ ./REPL[12]:1 [inlined]
  [9] _pullback(ctx::Zygote.Context, f::var"#9#10", args::Int64)
    @ Zygote ~/.julia/packages/Zygote/i1R8y/src/compiler/interface2.jl:0
 [10] _pullback(f::Function, args::Int64)
    @ Zygote ~/.julia/packages/Zygote/i1R8y/src/compiler/interface.jl:34
 [11] pullback(f::Function, args::Int64)
    @ Zygote ~/.julia/packages/Zygote/i1R8y/src/compiler/interface.jl:40
 [12] gradient(f::Function, args::Int64)
    @ Zygote ~/.julia/packages/Zygote/i1R8y/src/compiler/interface.jl:58
 [13] top-level scope
    @ REPL[12]:1

I think the solution is just to replace instances of T.mutable in the code with a backwards compatible call to ismutabletype. I can hazard a PR unless someone suggests a better solution.

[marius311]

Oops sorry I see its fixed on master 0.6.14 already.