switch to mercurial source control manager.

[williamstein]

Why: Because SAGE is huge and darcs doesn't scale sufficiently well.
Mercurial does scale to the task, though is less capable.

Component: basic arithmetic

Issue created by migration from https://trac.sagemath.org/ticket/74

[williamstein]

comment:1

This is now finished for the source code.
What remains:

• write hg_source, etc., python interfaces
• move doc, extcode, and scripts into mercurial.