strtod() and friends will happily swallow newlines, so we need to skip
over spaces and look for that sort of thing manually.

The bad line is printed out on a new line, and a third line
with a ^ to indicate the position of the error.

Fixes various corner cases and other bugs regarding lines with
comments, having data but no endpoints, etc.

This should more or less complete the rocket interface.

The server buffers the string and passes it to nilmdb.  Nilmdb passes
the string to bulkdata.  Bulkdata uses the rocket interface to parse
it in chunks, as necessary.  Everything gets passed back up and
everyone is happy.

Currently, only pyrocket implements append_string.

There isn't much left in File, so let's move as much as possible
over to C.

This is mostly a matter of taste, but it matches more closely with the
old way that prep did it, and it's more consistent.  It should roughly
match the available precision of floats and doubles.

This is maybe 2.5-3 times faster than the list-based code, which
still isn't amazing, but is decent.

Currently, only append_list is written (and hardly tested)