Change top-level package, improved doc

This release is rather simple, and motivating mostly by the breaking change
nature of renaming the module from minivariant.variant to geod24.variant.
The motivation is to avoid the proliferation of top-level package names
with single-purpose / single-module libraries.

Make index type depend on the Tuple size

Before this release, a variant of type Variant!(void*, uint, Object)
would have a size of 128 (64 + 64) bits on a 64 bits architecture,
which doubled the size of the variant for no reason.
With this release, the size will be 72 (64 + 8) bits.
While it doesn't fix a bug, it's a backward-compatible
improvement which deserves its own patch release.

Minivariant v1.0.0: First release

Minivariant is a simple library to provide tagged unions to the user.
The main utility is the Variant(T...) which is akin to std.variant.Algebraic but aims to provide a better API.
For example, functions overloads are used instead of a templated opAssign, and as such follows the usual conversion rules.

See the README and documentation for some usage example.