More statistics, list and string functions

[sharkdp]

This PR adds the following functions — all implemented in Numbat!

Statistics:

@name("Variance")
@url("https://en.wikipedia.org/wiki/Variance")
@description("Calculate the population variance of a list of quantities")
fn variance<D: Dim>(xs: List<D>) -> D^2

@name("Standard deviation")
@url("https://en.wikipedia.org/wiki/Standard_deviation")
@description("Calculate the population standard deviation of a list of quantities")
fn stdev<D: Dim>(xs: List<D>) -> D

@name("Median")
@url("https://en.wikipedia.org/wiki/Median")
@description("Calculate the median of a list of quantities")
fn median<D: Dim>(xs: List<D>) -> D

Lists:

@description("Get the first `n` elements of a list")
fn take<A>(n: Scalar, xs: List<A>) -> List<A>

@description("Get everything but the first `n` elements of a list")
fn drop<A>(n: Scalar, xs: List<A>) -> List<A>

@description("Get the element at index `i` in a list")
fn element_at<A>(i: Scalar, xs: List<A>) -> A

@description("Sort a list of quantities")
fn sort<D: Dim>(xs: List<D>) -> List<D>

Lists and strings:

@description("Convert a list of strings into a single string by concatenating them with a separator")
fn join(xs: List<String>, sep: String)

@description("Split a string into a list of strings using a separator")
fn split(input: String, separator: String) -> List<String>

Strings

fn str_find(haystack: String, needle: String) -> Scalar

related ticket: #144