Rewrite and merge linear regression asymptotics and heteroskedasticity

This commit moves the white-standard-errors material into the asymptotics
document and merges it with the linear regression asymptotics.  It also
rewrites the linear regression asymptotics to use sentences and paragraphs
instead of a loose outline.
Rewrite asymptotics results for general estimators

This commit moves the discussion of general asymptotic properties
of estimators into a single file.  It also converts the results into
paragraphs.