copyuntil: reduce over-allocation to start

This fits into a 32-byte allocation pool, saving up to 64 bytes when
repeatedly reading small chunks of data (e.g. tokenizing a CSV file). In
some local `@btime` measurements, this seems to take <10% more time
across a range of output lengths.

base/io.jl

@@ -543,8 +543,8 @@ julia> rm("my_file.txt")
 ```
 """
 readuntil(filename::AbstractString, delim; kw...) = open(io->readuntil(io, delim; kw...), convert(String, filename)::String)
-readuntil(stream::IO, delim::UInt8; kw...) = _unsafe_take!(copyuntil(IOBuffer(sizehint=70), stream, delim; kw...))
-readuntil(stream::IO, delim::Union{AbstractChar, AbstractString}; kw...) = String(_unsafe_take!(copyuntil(IOBuffer(sizehint=70), stream, delim; kw...)))
+readuntil(stream::IO, delim::UInt8; kw...) = _unsafe_take!(copyuntil(IOBuffer(sizehint=16), stream, delim; kw...))
+readuntil(stream::IO, delim::Union{AbstractChar, AbstractString}; kw...) = String(_unsafe_take!(copyuntil(IOBuffer(sizehint=16), stream, delim; kw...)))
 readuntil(stream::IO, delim::T; keep::Bool=false) where T = _copyuntil(Vector{T}(), stream, delim, keep)
 
 
@@ -617,7 +617,7 @@ Logan
 readline(filename::AbstractString; keep::Bool=false) =
     open(io -> readline(io; keep), filename)
 readline(s::IO=stdin; keep::Bool=false) =
-    String(_unsafe_take!(copyline(IOBuffer(sizehint=70), s; keep)))
+    String(_unsafe_take!(copyline(IOBuffer(sizehint=16), s; keep)))
 
 """
     copyline(out::IO, io::IO=stdin; keep::Bool=false)
@@ -1111,7 +1111,7 @@ function copyuntil(out::IO, io::IO, target::AbstractString; keep::Bool=false)
 end
 
 function readuntil(io::IO, target::AbstractVector{T}; keep::Bool=false) where T
-    out = (T === UInt8 ? resize!(StringVector(70), 0) : Vector{T}())
+    out = (T === UInt8 ? resize!(StringVector(16), 0) : Vector{T}())
     readuntil_vector!(io, target, keep, out)
     return out
 end