The elvdocs still use the old format (#elvdoc:fn or #elvdoc:var) for now, but
will be changed to "fn" and "var" forms soon.
Also remove the accidentally committed cmd/mvelvdoc. It has been used to perform
the conversion automatically but is not supposed to be committed.
Non-portable ones are moved into new packages pkg/sys/e{unix windows}. The "e"
prefix is needed to avoid conflict with packages under golang.org/x/sys/ and can
mean "extra".
Introduce `builtin:buildinfo` and `builtin:version` vars. This also
changes the `-buildinfo -json` implementation to use the standard
encoding/json package rather than handcrafting the JSON string.
This also fixes three incorrect spellings of "overridden". Normally I would
do those in a separate change but since there are only three instances,
one of which one was legitimately part of this change, I decided to bundle
the other two.
This also replaces the slightly awkward "arguments here" reason with
"argument count" as the "what" for a typical errs.ArityMismatch
exception. It also reformats most of the constructors so that the "what"
is on the same line. This makes `grep errs.ArityMismatch **.go` more
useful as a result.
This change is a preparation step for refining all *Op types to return Exception
as the error.
Keeping Exception as a struct type will make such a change error-prone, since
a (*Exception)(nil) != error(nil), so if an *Op returns a nil *Exception, it
is not nil if the return value is stored in an error-typed variable.
