common function and constant names

[kevinbarabash]

The common function names can be use by parsers that don't support multi-char identifiers except for common functions and constants, e.g. the parser from KAS.

We use the English names for these functions as identifiers. Software using math-ast should handle localization to and from these identifiers.

Functions:

• Trig functions: cos, sin, tan, acos, asin, atan, cosh, sinh, tanh, acosh, asinh, atanh
• Log functions: log (base 10 log), lg (base 2 log), ln (natural log)
• Other functions: n choose k, n permute k, nthroot, bessel functions, Gamma, ?

Constants:

• e, pi, tau, phi, ?