Various trigonometric computations require Angle constants of these values
but as construction via Angle::new is not possible in constants, this uses
fixed storage types and base units to directly construct Quantity literals
to wrap the standard libraries constants for π.

Add constants to Angle representing half and full turns

Added foot-candle unit to luminance

In Rust 1.45 the upgrade to LLVM 10.0 changes the behavior of
`f{32|64}::powi` on Windows. This commit changes the `Quantity::powi`
implementation to simplify down to `f{32|64}::powi` instead of using
`Typenum::Pow::powi` which uses a less precise algorithm. Resolves #192.

Add support for solid angles mostly based on what we do for angles.

Impls were problematic because they assume the value is always in
radians. Using explicit conversions that include the unit should be used
instead. Impls were likely added when `Angle` was added and adapted from
`Ratio`. Resolves #188.

mks and si examples are updated to use `format_args` or
`into_format_args` as appropriate so that the `Display` trait can be
used to display quantities in specific units. Part of #123.

Updated electronvolt definition, fixed KeV -> keV.

Add prefixes for electronvolt (eV).