make fallback signed and unsigned functions unchecked (as documented)

[JeffBezanson]

This helps e.g. BitIntegers.jl. convert is checked but signed and unsigned are not, so the latter should not be implemented using the former.

[KristofferC]

This made the tests for EzXML fail https://github.com/JuliaCI/NanosoldierReports/blob/f062a47c6bb3b5dd7c7bd29e7a3cc53cc6feeedd/pkgeval/by_hash/c32d166_vs_2e6715c/logs/EzXML/1.4.0-DEV-015cd56127.log#L64-L84

show: Error During Test at /home/pkgeval/.julia/packages/EzXML/QtGgF/test/runtests.jl:1304
  Test threw exception
  Expression: occursin(r"^EzXML.Node\(<DOCUMENT_NODE@0x[a-f0-9]+>\)$", repr(doc.node))
  MethodError: no method matching zero(::Ptr{EzXML._Node})
  Closest candidates are:
    zero(!Matched::Type{Missing}) at missing.jl:103
    zero(!Matched::Type{LibGit2.GitHash}) at /workspace/srcdir/usr/share/julia/stdlib/v1.4/LibGit2/src/oid.jl:220
    zero(!Matched::Type{Pkg.Resolve.VersionWeight}) at /workspace/srcdir/usr/share/julia/stdlib/v1.4/Pkg/src/Resolve/versionweights.jl:15
    ...
  Stacktrace:
   [1] unsigned(::Ptr{EzXML._Node}) at ./int.jl:158
   [2] show(::Base.GenericIOBuffer{Array{UInt8,1}}, ::EzXML.Node) at /home/pkgeval/.julia/packages/EzXML/QtGgF/src/node.jl:306
   [3] sprint(::Function, ::EzXML.Node; context::Nothing, sizehint::Int64) at ./strings/io.jl:105
   [4] repr(::EzXML.Node; context::Nothing) at ./strings/io.jl:227
   [5] repr(::EzXML.Node) at ./strings/io.jl:227
   [6] macro expansion at /home/pkgeval/.julia/packages/EzXML/QtGgF/test/runtests.jl:1304 [inlined]
   [7] macro expansion at /workspace/srcdir/usr/share/julia/stdlib/v1.4/Test/src/Test.jl:1116 [inlined]
   [8] macro expansion at /home/pkgeval/.julia/packages/EzXML/QtGgF/test/runtests.jl:1303 [inlined]
   [9] macro expansion at /workspace/srcdir/usr/share/julia/stdlib/v1.4/Test/src/Test.jl:1116 [inlined]
   [10] top-level scope at /home/pkgeval/.julia/packages/EzXML/QtGgF/test/runtests.jl:1302

which comes from a @printf: https://github.com/bicycle1885/EzXML.jl/blob/master/src/node.jl#L306