Base: PCRE: exec: type assert String after construction

[nsajko]

Should prevent some invalidation in the sysimage.

[nsajko]

Reduces the invalidation count on running struct I <: Integer end; Base.UInt(::I) = 0x7 from 71 to 65. Reproduce with:

./julia -E 'using SnoopCompileCore; i = (@snoop_invalidations (struct I <: Integer end; Base.UInt(::I) = 0x7)); using SnoopCompile; length(uinvalidated(i))'

Prevents these specific invalidations:

{   
    "method_instance": {
        "method": "_exec(re, subject, offset, options, match_data) @ Base.PCRE pcre.jl:204",
        "method_instance": "MethodInstance for Base.PCRE._exec(::Ptr{Nothing}, ::Any, ::Int64, ::UInt32, ::Ptr{Nothing})"
    },
    "children": [
        {   
            "method_instance": {
                "method": "exec(re, subject, offset, options, match_data) @ Base.PCRE pcre.jl:201",
                "method_instance": "MethodInstance for Base.PCRE.exec(::Ptr{Nothing}, ::Any, ::Int64, ::UInt32, ::Ptr{Nothing})"
            },
            "children": [
                {   
                    "method_instance": {
                        "method": "exec_r(re, subject, offset, options) @ Base.PCRE pcre.jl:213",
                        "method_instance": "MethodInstance for Base.PCRE.exec_r(::Ptr{Nothing}, ::Any, ::Int64, ::UInt32)"
                    },
                    "children": [
                        {   
                            "method_instance": {
                                "method": "var\"#occursin#463\"(offset::Integer, ::typeof(occursin), r::Regex, s::AbstractString) @ Base regex.jl:306",
                                "method_instance": "MethodInstance for Base.var\"#occursin#463\"(::Int64, ::typeof(occursin), ::Regex, ::AbstractString)"
                            },
                            "children": [
                                {   
                                    "method_instance": {
                                        "method": "occursin(r::Regex, s::AbstractString; offset) @ Base regex.jl:306",
                                        "method_instance": "MethodInstance for occursin(::Regex, ::AbstractString)"
                                    },
                                    "children": [
                                        {
                                            "method_instance": {
                                                "method": "url(m::Method) @ Base methodshow.jl:378",
                                                "method_instance": "MethodInstance for Base.url(::Method)"
                                            },
                                            "children": [                       
                                            ]           
                                        }
                                    ]
                                }
                            ]
                        }
                    ]
                }
            ]
        }
    ]
}