Support UUID and other stringlike types as keys

[treapster]

This PR fixes the following error with UUID and other types:

julia> using JSON, UUIDs, Dates

julia> JSON.json(Dict(uuid4() => "str"))
ERROR: ArgumentError: No key representation for UUID. Define StructUtils.lowerkey(::JSON.JSONStyle, ::UUID)
Stacktrace:
 [1] lowerkey(::JSON.JSONWriteStyle, x::UUID)
   @ JSON ~/.julia/packages/JSON/eypqd/src/write.jl:254
 [2] applyeach
   @ ~/.julia/packages/StructUtils/RnTYI/src/StructUtils.jl:578 [inlined]
 [3] json!(buf::Vector{…}, pos::Int64, x::Dict{…}, opts::JSON.WriteOptions{…}, ancestor_stack::Vector{…}, io::Nothing, ind::Int64, depth::Int64, bufsize::Int64)
   @ JSON ~/.julia/packages/JSON/eypqd/src/write.jl:671
 [4] json! (repeats 3 times)
   @ ~/.julia/packages/JSON/eypqd/src/write.jl:612 [inlined]
 [5] json(x::Dict{UUID, String}; pretty::Bool, kw::@Kwargs{})
   @ JSON ~/.julia/packages/JSON/eypqd/src/write.jl:482
 [6] json(x::Dict{UUID, String})
   @ JSON ~/.julia/packages/JSON/eypqd/src/write.jl:475
 [7] top-level scope
   @ REPL[3]:1
Some type informatio

The same error happens with Char, VersionNumber and some other types which should work out of the box.
For lowerkey I also changed Union{Float16, Float32, Float64} to Real, but if there is a reason why only specific types were supported we can revert it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.26%. Comparing base (f4fbb5a) to head (a9fccea).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #435   +/-   ##
=======================================
  Coverage   90.26%   90.26%           
=======================================
  Files           7        7           
  Lines        1366     1366           
=======================================
  Hits         1233     1233           
  Misses        133      133           

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