Add specifications for constants and associated element-wise functions

[kgryte]

This PR

• adds specifications for constants and associated element-wise functions.
• is derived from comparing API signatures across array libraries.

Notes

• Seems as though most libraries rely on NumPy for constants.
• This PR omits the euler_gamma constant, as it is rarely used according to record data.
• This PR includes one and only one alias for inf and nan.
• This PR does not include aliases for +0 and -0. The record data does not show their use.

[rgommers]

LGTM.

Constants like inf are not universally implemented, but are heavily used and it's quite weird that there are libraries that prefer users to spell that as float('Inf') instead.

[rgommers]

This is very straightforward, merging. Thanks @kgryte