  We present new stellar mass functions at $z\sim6$, $z\sim7$, $z\sim8$,
$z\sim9$ and, for the first time, $z\sim10$, constructed from $\sim800$
Lyman-Break galaxies previously identified over the XDF/UDF, parallels and the
five CANDELS fields. Our study is distinctive due to (1) the much deeper
($\sim200$ hour) wide-area Spitzer/IRAC imaging at $3.6\mu$m and $4.5\mu$m from
the GOODS Re-ionization Era wide Area Treasury from Spitzer (GREATS) program
and (2) consideration of $z\sim6-10$ sources over a $3\times$ larger area than
previous HST+Spitzer studies. The Spitzer/IRAC data enable $\ge2\sigma$
rest-frame optical detections for an unprecedented $50\%$ of galaxies down to a
stellar mass limit of $\sim10^{8}\mathcal{M}_\odot$ across all redshifts.
Schechter fits to our volume densities suggest a combined evolution in
characteristic mass $\mathcal{M}^*$ and normalization factor $\phi^*$ between
$z\sim6$ and $z\sim8$. The stellar mass density (SMD) increases by
$\sim1000\times$ in the $\sim500$ Myr between $z\sim10$ and $z\sim6$, with
indications of a steeper evolution between $z\sim10$ and $z\sim8$, similar to
the previously-reported trend of the star-formation rate density. Strikingly,
abundance matching to the Bolshoi-Planck simulation indicates halo mass
densities evolving at approximately the same rate as the SMD between $z\sim10$
and $z\sim4$. Our results show that the stellar-to-halo mass ratios, a proxy
for the star-formation efficiency, do not change significantly over the huge
stellar mass build-up occurred from $z\sim10$ to $z\sim6$, indicating that the
assembly of stellar mass closely mirrors the build-up in halo mass in the first
$\sim1$ Gyr of cosmic history. JWST is poised to extend these results into the
"first galaxy" epoch at $z\gtrsim10$.
