`str.repr` and related functions

[CoffeeTableEspresso]

these should return a string representation that is legal YASL code.

for example:

const x = 'string'
echo x           # string
echo x->repr()   # 'string'

Strings will be quoted, certain characters will be escaped, etc.

list.repr and table.repr will behave recursively.

Possibly, we need a set.repr as well.

[CoffeeTableEspresso]

For many types, these could defer to type.tostr, for example booleans and ints.

[CoffeeTableEspresso]

Added a parameter to tostr in v0.13.0