Number files like chapters. Consolidate toc & preface.

Aside from the consolidation of title & preface in index.md,
this commit was produced as follows:

```
cd spec/

g mv 03-lexical-syntax.md                      01-lexical-syntax.md
g mv 04-identifiers-names-and-scopes.md        02-identifiers-names-and-scopes.md
g mv 05-types.md                               03-types.md
g mv 06-basic-declarations-and-definitions.md  04-basic-declarations-and-definitions.md
g mv 07-classes-and-objects.md                 05-classes-and-objects.md
g mv 08-expressions.md                         06-expressions.md
g mv 09-implicit-parameters-and-views.md       07-implicit-parameters-and-views.md
g mv 10-pattern-matching.md                    08-pattern-matching.md
g mv 11-top-level-definitions.md               09-top-level-definitions.md
g mv 12-xml-expressions-and-patterns.md        10-xml-expressions-and-patterns.md
g mv 13-user-defined-annotations.md            11-user-defined-annotations.md
g mv 14-the-scala-standard-library.md          12-the-scala-standard-library.md
g mv 15-syntax-summary.md                      13-syntax-summary.md
g mv 16-references.md                          14-references.md

perl -pi -e 's/03-lexical-syntax/01-lexical-syntax/g' *.md
perl -pi -e 's/04-identifiers-names-and-scopes/02-identifiers-names-and-scopes/g' *.md
perl -pi -e 's/05-types/03-types/g' *.md
perl -pi -e 's/06-basic-declarations-and-definitions/04-basic-declarations-and-definitions/g' *.md
perl -pi -e 's/07-classes-and-objects/05-classes-and-objects/g' *.md
perl -pi -e 's/08-expressions/06-expressions/g' *.md
perl -pi -e 's/09-implicit-parameters-and-views/07-implicit-parameters-and-views/g' *.md
perl -pi -e 's/10-pattern-matching/08-pattern-matching/g' *.md
perl -pi -e 's/11-top-level-definitions/09-top-level-definitions/g' *.md
perl -pi -e 's/12-xml-expressions-and-patterns/10-xml-expressions-and-patterns/g' *.md
perl -pi -e 's/13-user-defined-annotations/11-user-defined-annotations/g' *.md
perl -pi -e 's/14-the-scala-standard-library/12-the-scala-standard-library/g' *.md
perl -pi -e 's/15-syntax-summary/13-syntax-summary/g' *.md
perl -pi -e 's/16-references/14-references/g' *.md
```
Stubs for references chapter, remains TODO
SI-6069 Preserve by-name during eta-expansion

This wording attempts to describe the behavior change in SI-5610
while remaining non-committal about the mechanism.

It refers to the section on function applications and attempts to
use similar language.
SI-6054 Modern eta-expansion examples that almost run

This wasn't so easy after all. The clever one is to show
that `Option.fold(=>Z)` preserves the by-nameness of the zero value,
which may or may not be in the spec yet.

The addition to 6.26.5 was (I think) rejected as implementation-specific;
it should use the wording from 6.6 about "preserving by-nameness" or "by-namity"
(the latter has the advantage of sounding both more technical and somewhat inane).