Add definitions for internal TOML methods in Base

Despite being internal, these interfaces are used in a number of places
downstream, so it's best for TOML's parser type to have their interface
defined.

stdlib/TOML/src/TOML.jl

@@ -25,7 +25,7 @@ module Internals
 end
 
 # https://github.com/JuliaLang/julia/issues/36605
-readstring(f::AbstractString) = isfile(f) ? read(f, String) : error(repr(f), ": No such file")
+_readstring(f::AbstractString) = isfile(f) ? read(f, String) : error(repr(f), ": No such file")
 
 """
     Parser()
@@ -55,9 +55,9 @@ Parse file `f` and return the resulting table (dictionary). Throw a
 See also [`TOML.tryparsefile`](@ref).
 """
 parsefile(f::AbstractString) =
-    Internals.parse(Internals.Parser{Dates}(readstring(f); filepath=abspath(f)))
+    Internals.parse(Internals.Parser{Dates}(_readstring(f); filepath=abspath(f)))
 parsefile(p::Parser, f::AbstractString) =
-    Internals.parse(Internals.reinit!(p._p, readstring(f); filepath=abspath(f)))
+    Internals.parse(Internals.reinit!(p._p, _readstring(f); filepath=abspath(f)))
 
 """
     tryparsefile(f::AbstractString)
@@ -69,9 +69,9 @@ Parse file `f` and return the resulting table (dictionary). Return a
 See also [`TOML.parsefile`](@ref).
 """
 tryparsefile(f::AbstractString) =
-    Internals.tryparse(Internals.Parser{Dates}(readstring(f); filepath=abspath(f)))
+    Internals.tryparse(Internals.Parser{Dates}(_readstring(f); filepath=abspath(f)))
 tryparsefile(p::Parser, f::AbstractString) =
-    Internals.tryparse(Internals.reinit!(p._p, readstring(f); filepath=abspath(f)))
+    Internals.tryparse(Internals.reinit!(p._p, _readstring(f); filepath=abspath(f)))
 
 """
     parse(x::Union{AbstractString, IO})
@@ -135,4 +135,17 @@ supported type.
 """
 const print = Internals.Printer.print
 
+public Parser, parsefile, tryparsefile, parse, tryparse, ParserError, print
+
+# These methods are private Base interfaces, but we do our best to support them over
+# the TOML stdlib types anyway to minimize downstream breakage.
+Base.TOMLCache(p::Parser) = Base.TOMLCache(p._p, Dict{String, Base.CachedTOMLDict}())
+Base.TOMLCache(p::Parser, d::Base.CachedTOMLDict) = Base.TOMLCache(p._p, d)
+Base.TOMLCache(p::Parser, d::Dict{String, Dict{String, Any}}) = Base.TOMLCache(p._p, d)
+
+Internals.reinit!(p::Parser, str::String; filepath::Union{Nothing, String}=nothing) =
+    Internals.reinit!(p._p, str; filepath)
+Internals.parse(p::Parser) = Internals.parse(p._p)
+Internals.tryparse(p::Parser) = Internals.tryparse(p._p)
+
 end