[lex] Complete the use of unicode code points

Completes the task of applying Unicode code point markup to
refer to single characters in normative text.

Replace all remaining uses of backslash as glyph (\) or text
with the corresponding `unicode{005c}{reverse solidus}` markup.

Replace a couple of text reference to single and double quotes
with their corresponding Unicode markup, and similarly for the
glyphs ` and ".

This should compete the last Unicode markup in [lex].

Author: AlisdairM

source/lex.tex

@@ -99,12 +99,12 @@
 \indextext{line splicing}%
 If the first translation character is \unicode{feff}{byte order mark},
 it is deleted.
-Each sequence comprising a backslash character (\textbackslash)
+Each sequence comprising a \unicode{005c}{reverse solidus} character (\textbackslash)
 immediately followed by
 zero or more whitespace characters other than new-line followed by
 a new-line character is deleted, splicing
 physical source lines to form \defnx{logical source lines}{source line!logical}. Only the last
-backslash on any physical source line shall be eligible for being part
+ \unicode{005c}{reverse solidus} on any physical source line shall be eligible for being part
 of such a splice.
 \begin{note}
 Line splicing can form
@@ -576,7 +576,8 @@
 circumstances during translation phase 4, whitespace (or the absence
 thereof) serves as more than preprocessing token separation. Whitespace
 can appear within a preprocessing token only as part of a header name or
-between the quotation characters in a character literal or
+between the \unicode{0027}{apostrophe} characters in a character literal
+or between the \unicode{0022}{quotation mark} characters in a
 string literal.
 
 \pnum
@@ -727,13 +728,14 @@
 \end{note}
 
 \pnum
-The appearance of either of the characters \tcode{'} or \tcode{\textbackslash} or of
+The appearance of either of the characters \unicode{0027}{apostrophe} or
+\unicode{005c}{reverse solidus} or of
 either of the character sequences \tcode{/*} or \tcode{//} in a
 \grammarterm{q-char-sequence} or an \grammarterm{h-char-sequence}
 is conditionally-supported with \impldef{meaning of \tcode{'}, \tcode{\textbackslash},
 \tcode{/*}, or \tcode{//} in a \grammarterm{q-char-sequence} or an
 \grammarterm{h-char-sequence}} semantics, as is the appearance of the character
-\tcode{"} in an \grammarterm{h-char-sequence}.
+\unicode{0022}{quotation mark} in an \grammarterm{h-char-sequence}.
 \begin{note}
 Thus, a sequence of characters
 that resembles an escape sequence can result in an error, be interpreted as the