Add methods of first and last accepting a predicate

On zulip, there was a thread about the fact that there's not a good
way of finding the first item in a non-indexable iterable without
manually writing out the for-loop. One possibility is using the
existing first function composed with Iterators.filter:

    first(Iterators.filter(>(5), 1:10))

The problem with this approach is that it requires wrapping in a try
block if the predicate not being satisfied is a
possibility (Iterators.filter returns an empty collection). There
should be a way of expressing this without using exception-handling as
control flow or writing the for-loop manually.

Ideally, I think first and last should just return
Union{Some{T},Nothing} always, but that would be a breaking
change. So I see two obvious alternatives:

1. This commit adds an optional predicate argument to first and last
   which is mapped over the collection and the first/last value
   satisfying predicate is returned.
2. Add a Iterators.first/Iterators.last which are identical to the
   versions in Base except that when successful they return Some{T}
   and when unsuccesful return Nothing

Author: non-Jedi

base/abstractarray.jl

@@ -343,6 +343,34 @@ function first(itr)
     x[1]
 end
 
+"""
+    first(predicate, coll)
+
+Get the first element of `coll` satisfying `predicate` wrapped in `Some`.
+
+If no element of `coll` satisfies `predicate`, return `nothing`.
+
+# Examples
+```jldoctest
+julia> first(>(5), 1:10)
+Some(6)
+
+julia> first(isodd, 2:2:10) |> isnothing
+true
+
+julia> first(iseven, [5; 3; 4; 2; 6; 8]) |> something
+4
+
+julia> something(first(>(10), 1:10), 0)
+0
+```
+"""
+function first(predicate, itr)
+    for x in itr
+        predicate(x) && return Some(x)
+    end
+end
+
 """
     first(itr, n::Integer)
 
@@ -388,6 +416,43 @@ julia> last([1; 2; 3; 4])
 """
 last(a) = a[end]
 
+"""
+    last(predicate, coll)
+
+Get the last element of `coll` satisfying `predicate` wrapped in `Some`.
+
+If no element of `coll` satisfies `predicate`, return `nothing`.
+
+# Examples
+```jldoctest
+julia> last(<(5), 1:10)
+Some(4)
+
+julia> last(isodd, 2:2:10) |> isnothing
+true
+
+julia> last(iseven, [5; 3; 4; 2; 6; 9]) |> something
+6
+
+julia> last(first(>(10), 1:10), 0)
+0
+```
+"""
+function last(predicate, itr)
+    out = nothing
+    for x in itr
+        out = ifelse(predicate(x), Some(x), out)
+    end
+    out
+end
+
+# faster version for arrays
+function last(predicate, a::AbstractArray)
+    @inbounds for i in reverse(eachindex(a))
+        predicate(a[i]) && return Some(a[i])
+    end
+end
+
 """
     last(itr, n::Integer)
 

test/abstractarray.jl

@@ -1141,3 +1141,13 @@ end
     @test last(itr, 1) == [itr[end]]
     @test_throws ArgumentError last(itr, -6)
 end
+
+@testset "first/last element satisfying predicate of $(typeof(itr))" for itr in (1:9,
+                                                                                 collect(1:9),
+                                                                                 reshape(1:9, (3, 3)),
+                                                                                 ntuple(identity, 9))
+    @test first(>(5), itr) |> something == 6
+    @test last(<(5), itr) |> something == 4
+    @test first(>(9), itr) == nothing
+    @test last(>(9), itr) == nothing
+end