strconv: should make Decimal private?

[liuly0322]

i.e.

patch file

diff --git a/strconv/decimal.mbt b/strconv/decimal.mbt
index 4aa7b2f..daa8c3e 100644
--- a/strconv/decimal.mbt
+++ b/strconv/decimal.mbt
@@ -19,7 +19,7 @@
/// reference:
/// - <https://nigeltao.github.io/blog/2020/parse-number-f64-simple.html>
/// - <https://nigeltao.github.io/blog/2020/eisel-lemire.html>
-struct Decimal {
+priv struct Decimal {
  digits : Bytes
  mut digits_num : Int
  mut decimal_point : Int
@@ -41,7 +41,7 @@ let powtab = [

///|
/// Create a zero decimal.
-pub fn Decimal::new() -> Decimal {
+fn Decimal::new() -> Decimal {
  {
    digits: Bytes::new(800),
    digits_num: 0,
@@ -53,13 +53,13 @@ pub fn Decimal::new() -> Decimal {

///|
/// Create a decimal with an Int64 value.
-pub fn Decimal::from_int64(v : Int64) -> Decimal {
+fn Decimal::from_int64(v : Int64) -> Decimal {
  Decimal::new()..assign(v)
}

///|
/// Create a decimal from number string.
-pub fn Decimal::parse_decimal(str : String) -> Decimal!StrConvError {
+fn Decimal::parse_decimal(str : String) -> Decimal!StrConvError {
  let d = Decimal::new()
  if str.length() <= 0 {
    syntax_err!()
@@ -145,7 +145,7 @@ pub fn Decimal::parse_decimal(str : String) -> Decimal!StrConvError {

///|
/// Convert the decimal to Double.
-pub fn to_double(self : Decimal) -> Double!StrConvError {
+fn to_double(self : Decimal) -> Double!StrConvError {
  let mut exponent = 0
  let mut mantissa = 0L
  // check the underflow and overflow
@@ -231,7 +231,7 @@ pub fn to_double(self : Decimal) -> Double!StrConvError {
///|
/// Binary shift left (s > 0) or right (s < 0).
/// The shift count must not larger than the max_shift to avoid overflow.
-pub fn shift(self : Decimal, s : Int) -> Unit {
+fn shift(self : Decimal, s : Int) -> Unit {
  if self.digits_num == 0 {
    return
  }

Why

1. strconv should only provide:
   

   core/strconv/README.md

   Line 9 in bcb83ee

   Use `parse_bool`, `parse_double`, `parse_int`, and `parse_int64` convert strings to values.

2. The current Decimal is only used for parse_double. Go also only uses it internally: https://github.com/golang/go/blob/go1.15.3/src/strconv/decimal.go#L6
3. The current Decimal implementation is only correct for internal usage:

   let d = @strconv.Decimal::from_int64(-1)
   let e = match d.to_double?() {
     Ok(x) => x
     Err(_) => {
       println("error")
       0.0
     }
   }
   println(e)

   The above user code will print 0, as negative sign is not handled properly.

Willing to make a PR.