[intro.scope, namespace.udecl, temp.deduct.type] Avoid "and so" (#1030)

* [intro.scope] Change "and so" to "so"

* [namespace.udecl] Rephrase a sentence

* [temp.deduct.type] Change "and so" to "so"

Author: Johannes Laire

source/declarations.tex

@@ -2697,7 +2697,7 @@
 
 \pnum
 Every \grammarterm{using-declaration} is a \grammarterm{declaration} and a
-\grammarterm{member-declaration} and so can be used in a class definition.
+\grammarterm{member-declaration} and can therefore be used in a class definition.
 \begin{example}
 
 \begin{codeblock}

source/intro.tex

@@ -31,7 +31,7 @@
 \indextext{scope|(}%
 This International Standard specifies requirements for implementations
 of the \Cpp programming language. The first such requirement is that
-they implement the language, and so this International Standard also
+they implement the language, so this International Standard also
 defines \Cpp. Other requirements and relaxations of the first
 requirement appear at various places within this International Standard.
 

source/templates.tex

@@ -6846,7 +6846,7 @@
 \begin{note}
 Under \ref{temp.deduct.call} and \ref{temp.deduct.partial},
 if \tcode{P} contains no \grammarterm{template-parameter}{s} that appear
-in deduced contexts, no deduction is done, and so \tcode{P} and \tcode{A}
+in deduced contexts, no deduction is done, so \tcode{P} and \tcode{A}
 need not have the same form.
 \end{note}