updating read for StructTypes.Mutable()

[Aaron Matthis]

I have a situation like that:

    Base.@kwdef mutable struct PublisherConfig
        CIName::String = "Test"
        CIFileName::String = "CI.yml"
        SSHSecretName::String = "SSH_private_access"
    end
    JSON3.StructType(::Type{PublisherConfig}) = JSON3.StructTypes.Mutable()
    const CONFIG = PublisherConfig()

Now, the assignment in the last line is important because it shall be a const. the const is important for performance. Thus, what I'm missing is an option to update the pre-existing object from JSON3 (rather than having JSON3 create a new one)

[Jacob Quinn]

Ah, that does seem useful. I don't think it would actually be very hard to support; we could introduce a JSON3.read!(json, x::T) public function to essentially "populate x" from fields found in json. Internally, it would also not be a ton of work, because we'd make the main JSON3.read for mutables take an existing x::T, which JSON3.read! would call directly, and JSON3.read would just create a x = T() and pass it to the same. Feel like taking a stab at it?

[Dilum Aluthge]

Implemented in #79