constants 1.0.0

Update to version 8.1, the 2018 CODATA recommended values (#7 addressing #6).
This version contains some breaking changes that are necessary to streamline
future updates and provide a stable symbol table:

• The codata table includes the absolute uncertainty instead of the relative
  one. Thus, the rel_uncertainty column has been dropped in favour of the new
  uncertainty. Also, columns have been slightly reordered.
• Symbol names for constants have changed. The old ones were hand-crafted and
  thus unmanageable. This release adopts the ASCII symbols defined by NIST in
  their webpage, except for those that collide with some base R function. In
  particular, there are two cases: c, the speed of light, has been renamed as
  c0; sigma, the Stefan-Boltzmann constant, has been renamed as sigma0.
• Constant types, or categories, (column codata$type) adopts the names defined
  by NIST in the webpage too. Some constants belong to more than one category
  (separated by comma); some others belong to no category (missing type).

There are some new features too:

• In addition to the codata data frame, this release includes codata.cor, a
  correlation matrix for all the constants.
• In addition to syms_with_errors and syms_with_units, there is a new list
  of symbols called syms_with_quantities (available if the optional
  quantities package is installed), which provides constant values with
  uncertainty and units.
• Experimental support for correlated values in syms_with_errors and
  syms_with_quantities is provided (disabled by default; see details in
  help(syms) for activation instructions).