ivy: add binary sys to allow setting of values

Been meaning to do this for a while.

Also add a sys op for "prec", which was overlooked.

Also fix lib/libgen.go, which was broken by the state change
in the previous CL.

ivy: fix version string

No matter how much care I take, some stupid error always slips through.

Fixes robpike#255

ivy: tag v4.0.3

ivy: add 'sys "trace" args...'

Allow a program to print a stack trace and continue execution.
Handy for debugging.

ivy: add support for libraries

A new special command allows one to execute the contents of a
pre-loaded library file.

	)lib foo

loads (executes) library foo.  If no library name is provided, all
available libraries are listed by name.

Even if the library is not loaded,

	)lib doc foo
Shows documentation for the library foo, at the moment defined to
be the leading comment in the file.

	)lib ops foo

lists the user-defined operators in the library and

	)lib vars foo

lists its global variables.

Libraries may be added by adding an ivy file to lib/lib and running
go generate in lib.

At the moment there is only a single library, a nutty pile of code
that tracks values and units of physical constants such as mass,
velocity, and so on. That library, called units, still needs a lot
of work but that can develop separately.

Also clean up a few gofmt issues. What, gofmt does smart quotes
now? Give me a break.

ivy: use box-drawing characters for multiline matrices and vectors (r…

…obpike#248)

Use Unicode box-drawing characters instead of pipes and parens.

Matrices now print using square brackets; vectors use rounded brackets.
At top level, matrices and vectors have no box, as always.
That is, iota 5 still prints as "1 2 3 4 5" not "(1 2 3 4 5)".
Boxes appear exactly when they did before,
just with Unicode characters instead of ASCII characters.

See the testdata changes for examples of the visual changes.

Fixes robpike#236.

ivy: allow () to parse as the empty vector

May roll this back if it causes trouble, but it's very easy and feels right.

Fixes robpike#246

ivy: rewrite multiline formatting (robpike#244)

ivy: fix missing paren in )save

A couple of early returns in the save code could cause it to
omit the closing parenthesis. For example,

	x = 'abc' 'def'

would save as

	x = ("abc" ("def"

Silly but easily fixed with a change to use 'break'.

ivy: delete unused code from value/log.go

The previous incorrect code was using the partial calculation of a failed
integer log, but in the rewrite that was never passed on, so delete that.

Again, keep it simple.