[P1] No way to parse a string to a number

[ecto]

Today there is no string to number conversion. [int "42"] and
[float "3.14"] both return (); there is no parse-int,
parse-float, to-int, or equivalent in the builtin set.

This is the single most surprising gap when writing real programs:
you cannot read a number from stdin, args, env, or a file.

Proposal

Add parse-int and parse-float, both returning Option
(or Result for richer errors):

[parse-int "42"]   ; [Some 42]
[parse-int "x"]    ; None
[parse-float "3.14"]

Acceptance criteria

• Builtins available in the VM and listed in web/src/pages/ref/builtins.loon.
• A test covering valid input, invalid input, leading/trailing whitespace,
  negative numbers, and out-of-range integers.

Found while completing the hyperpolyglot ml column.