Implement Mini operator functions

A PR in the stencila/mini repo implements operators as function calls https://github.com/stencila/mini/pull/18. These functions need to be implemented in libcore. Awaiting FunctionJsDocConverter to be implemented https://github.com/stencila/convert/pull/2 before doing this.

Precedence	Operator	Usage example	Function call equivalent
1	`.`	`value.member`	`select(value, member)`
1	`[]`	`value[member]`	`select(value, member)`
2	`!`	`!value`	`not(value)`
2	`+`	`+value`	`positive(value)`
2	`-`	`-value`	`negative(value)`
3	`^`	`value ^ expon`	`pow(value, expon)`
4	`*`	`value * other`	`multiply(value, other)`
4	`/`	`value / other`	`divide(value, other)`
4	`%`	`value % other`	`remainder(value, other)`
5	`+`	`value + other`	`add(value, other)`
5	`-`	`value - other`	`subtract(value, other)`
6	`<`	`value < other`	`less(value, other)`
6	`<=`	`value <= other`	`less_or_equal(value, other)`
6	`>`	`value > other`	`greater(value, other)`
6	`>=`	`value >= other`	`greater_or_equal(value, other)`
7	`==`	`value == other`	`equal(value, other)`
7	`!=`	`value != other`	`not_equal(value, other)`
8	`&&`	`value && other`	`and(value, other)`
9	`\\|\\|`	`value \\|\\| other`	`or(value, other)`

stencila / libcore