add std.contract.Sequence

[Radvendii]

This new function isn't so useful for applying directly, but there;s currently no way to store multiple contracts in a variable to be applied later. In other words

'foo | [| 'foo |]

is to

let Foo = [| 'foo |] in
'foo | Foo

as

"foo" | std.contract.TagOrString | [| 'foo |]

is to
???

Well, before there were no options. with this PR we can do

let Foo = std.contract.Sequence [ std.contract.TagOrString, [| 'foo |] ] in
"foo" | Foo

Eventually, it would be nice if the LSP could recognize this and get the same niceties as if you'd directly applied the two contracts.

[Radvendii]

related to #461

[vkleen]

I like it, but I may be biased 😅