[Belt] Stricter parse for Int and Float

The actual implementation of Belt.Int.fromString relies on the
Javascript parseInt function [0].  That function parses Strings in a
permissive manner.  For instance, if the function encounters an
alphabetical character, it returns the value up to that character:

  parseInt("123a456") // output: 123

The same applies for Belt.Float.fromString that relies on
parseFloat [1] and have a similar interpretation.

This commit proposes a stricter implementation of Int and Float parsing
using the Javascript Number constructor [2].  The Belt.Int.fromString
uses the more specific Math.trunc function [3], that parses string
similarly to Number and then returns the Int part of that number.

Relying only on the Number constructor keeps the implementation simple.
However, there exists one last quirk while using it with an empty
string.  It returns the 0 value:

  Number("") // ouptput: 0

Consequently, Belt.Int.fromString("") will return 0.  Tests for
Belt.(Int/Float).fromString have been updated in order to reflect that.

Fix rescript-lang#3732

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt
[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat
[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/Number
[3] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/trunc

Author: rcherrueau

jscomp/others/belt_Float.ml

@@ -32,7 +32,7 @@ external toInt: float -> int = "%intoffloat"
 
 external fromInt: int -> float = "%identity"
 
-external fromString: string -> float = "parseFloat" [@@bs.val]
+external fromString: string -> float = "Number" [@@bs.val]
 
 let fromString i =
   match (fromString i) with

jscomp/others/belt_Int.ml

@@ -32,7 +32,7 @@ external toFloat: int -> float = "%identity"
 
 external fromFloat: float -> int = "%intoffloat"
 
-external fromString: string -> (_ [@bs.as 10]) -> int = "parseInt" [@@bs.val]
+external fromString: string -> (_ [@bs.as 10]) -> int = "Math.trunc" [@@bs.val]
 
 let fromString i =
   match fromString i with

jscomp/test/bs_float_test.ml

@@ -29,7 +29,16 @@ let () =
   eq __LOC__ (F.fromString "-1.0") (Some (-1.0));
   eq __LOC__ (F.fromString "-1.5") (Some (-1.5));
   eq __LOC__ (F.fromString "-1.7") (Some (-1.7));
-  eq __LOC__ (F.fromString "not a float") None
+  eq __LOC__ (F.fromString "17e-1") (Some (1.7));
+  eq __LOC__ (F.fromString "-17e-1") (Some (-1.7));
+  eq __LOC__ (F.fromString "  -17e-1  ") (Some (-1.7));
+  eq __LOC__ (F.fromString "0x11") (Some (17.));
+  eq __LOC__ (F.fromString "0b11") (Some (3.));
+  eq __LOC__ (F.fromString "0o11") (Some (9.));
+  eq __LOC__ (F.fromString "") (Some (0.));
+  eq __LOC__ (F.fromString "not a float") None;
+  eq __LOC__ (F.fromString "100.0abcdef") None;
+  eq __LOC__ (F.fromString "123_456.7") None
 
 let () =
   eq __LOC__ (F.toString 1.0) "1";

jscomp/test/bs_int_test.ml

@@ -29,7 +29,16 @@ let () =
   eq __LOC__ (I.fromString "-1.0") (Some (-1));
   eq __LOC__ (I.fromString "-1.5") (Some (-1));
   eq __LOC__ (I.fromString "-1.7") (Some (-1));
-  eq __LOC__ (I.fromString "not an int") None
+  eq __LOC__ (I.fromString "17e-1") (Some (1));
+  eq __LOC__ (I.fromString "-17e-1") (Some (-1));
+  eq __LOC__ (I.fromString "  -17e-1  ") (Some (-1));
+  eq __LOC__ (I.fromString "0x11") (Some (17));
+  eq __LOC__ (I.fromString "0b11") (Some (3));
+  eq __LOC__ (I.fromString "0o11") (Some (9));
+  eq __LOC__ (I.fromString "") (Some (0));
+  eq __LOC__ (I.fromString "not an int") None;
+  eq __LOC__ (I.fromString "100abcdef") None;
+  eq __LOC__ (I.fromString "123_456") None
 
 let () =
   eq __LOC__ (I.toString 1) "1";