Silence deprecation warnings from string and escape_string (#204)

These functions print warnings because CategoricalString is still an AbstractString,
but the methods themselves are not deprecated, and the warnings will go away once
inheritance is removed. This is particularly visible when working with DataFrames.

src/deprecated.jl

@@ -93,4 +93,9 @@ import Unicode: normalize, graphemes
 @deprecate chomp(x::CategoricalString) chomp(String(x))
 @deprecate textwidth(x::CategoricalString) textwidth(String(x))
 @deprecate isascii(x::CategoricalString) isascii(String(x))
-@deprecate escape_string(x::CategoricalString) escape_string(String(x))
+@deprecate escape_string(x::CategoricalString) escape_string(String(x))
+
+# Avoid printing a deprecation until CategoricalString is no longer AbstractString
+Base.write(io::IO, x::CategoricalString) = write(io, get(x))
+Base.escape_string(io::IO, x::CategoricalString, esc) = escape_string(io, get(x), esc)
+Base.tostr_sizehint(x::CategoricalString) = Base.tostr_sizehint(get(x))