Serialization: fix a handful of implicit return mistakes

This would have caused us to perform a number of unncessary allocations and method specializations

base/mpfr.jl

@@ -38,6 +38,7 @@ function __init__()
  catch ex
  Base.showerror_nostdio(ex, "WARNING: Error during initialization of module MPFR")
  end
+ nothing
 end
 
 const ROUNDING_MODE = RefValue{Cint}(0)

stdlib/Serialization/src/Serialization.jl

@@ -137,15 +137,16 @@ const REF_OBJECT_TAG = Int32(o0+13)
 const FULL_GLOBALREF_TAG = Int32(o0+14)
 const HEADER_TAG = Int32(o0+15)
 
-writetag(s::IO, tag) = write(s, UInt8(tag))
+writetag(s::IO, tag) = (write(s, UInt8(tag)); nothing)
 
 function write_as_tag(s::IO, tag)
  tag < VALUE_TAGS && write(s, UInt8(0))
  write(s, UInt8(tag))
+ nothing
 end
 
 # cycle handling
-function serialize_cycle(s::AbstractSerializer, x)
+function serialize_cycle(s::AbstractSerializer, @nospecialize(x))
  offs = get(s.table, x, -1)::Int
  if offs != -1
  if offs <= typemax(UInt16)
@@ -225,6 +226,7 @@ function serialize(s::AbstractSerializer, x::Symbol)
  write(s.io, Int32(len))
  end
  unsafe_write(s.io, pname, len)
+ nothing
 end
 
 function serialize_array_data(s::IO, a)
@@ -290,6 +292,7 @@ function serialize(s::AbstractSerializer, ss::String)
  write(s.io, Int64(len))
  end
  write(s.io, ss)
+ nothing
 end
 
 function serialize(s::AbstractSerializer, ss::SubString{String})
@@ -546,6 +549,7 @@ function serialize_type_data(s, t::DataType)
  end
  end
  end
+ nothing
 end
 
 function serialize(s::AbstractSerializer, t::DataType)
@@ -575,6 +579,7 @@ function serialize(s::AbstractSerializer, n::Int32)
  writetag(s.io, INT32_TAG)
  write(s.io, n)
  end
+ nothing
 end
 
 function serialize(s::AbstractSerializer, n::Int64)
@@ -587,6 +592,7 @@ function serialize(s::AbstractSerializer, n::Int64)
  writetag(s.io, INT64_TAG)
  write(s.io, n)
  end
+ nothing
 end
 
 serialize(s::AbstractSerializer, ::Type{Bottom}) = write_as_tag(s.io, BOTTOM_TAG)
@@ -636,6 +642,7 @@ function serialize_any(s::AbstractSerializer, @nospecialize(x))
  end
  end
  end
+ nothing
 end
 
 """