Minor inference improvements in stdlibs

Since Julia unfortuantely still cannot propagate information from immutable
fields loaded twice (see JuliaLang#39888, JuliaLang#41199), these kind of changes are necessary
to achieve type stability.

stdlib/REPL/src/REPL.jl

@@ -467,7 +467,7 @@ mutable struct LineEditREPL <: AbstractREPL
             in_help,envcolors,false,nothing, opts, nothing, Tuple{String,Int}[])
     end
 end
-outstream(r::LineEditREPL) = r.t isa TTYTerminal ? r.t.out_stream : r.t
+outstream(r::LineEditREPL) = (t = r.t; t isa TTYTerminal ? t.out_stream : t)
 specialdisplay(r::LineEditREPL) = r.specialdisplay
 specialdisplay(r::AbstractREPL) = nothing
 terminal(r::LineEditREPL) = r.t

stdlib/Serialization/src/Serialization.jl

@@ -364,7 +364,8 @@ function serialize_mod_names(s::AbstractSerializer, m::Module)
     p = parentmodule(m)
     if p === m || m === Base
         key = Base.root_module_key(m)
-        serialize(s, key.uuid === nothing ? nothing : key.uuid.value)
+        uuid = key.uuid
+        serialize(s, uuid === nothing ? nothing : uuid.value)
         serialize(s, Symbol(key.name))
     else
         serialize_mod_names(s, p)