Replaced ad hoc internal data structures w/ SQLite.

This is the first step of a refactoring/limited-scope rewrite
that should improve performance and simplify operations such as
adding pages that contain posts with a certain tag. The code is
still unformatted and old comments may be out of date. There are
regressions to be fixed in this first version.

Note that this is a breaking change. Some user templates will
stop working until their procs are updated to match those
in default-procs.tcl in the project skeleton.
Renamed namescape 'database' 'pages'.

Shorted proc names within the namespace.
Line lengths. Renamed databased 'tclssg-db'.
Removed dict-sort tests.
Fixed post order in index, tag pages, sidebar.

hideFromSiderbar is no longer ignored.
Fixed non-link article titles in collections.

Updated required package list in README. Build results should now
be identical to those of v0.14.1.
Fixed proc local-image.
Store SQLite DB in :memory:. copy-links proc.

Moved the code for NS pages to the top of the file. Wrote and
updated comments.

The use of copy-links improves performance significantly for
larger websites (as indicated by tests with 2000 input files).
Better feedback from deploy-custom.

Updated output sample in README.
Fixed index, tag page duplication.