Remove 64-bit limit for base-62-numbers.

Demanglers should be prepared for any arbitrary length number.

Author: ehuss

src/doc/rustc/src/symbol-mangling/v0.md

@@ -1006,7 +1006,7 @@ A *lower* and *upper* is an ASCII lower and uppercase letter respectively.
 
 > [base-62-number] → { *[digit]* | *[lower]* | *[upper]* } `_`
 
-A *base-62-number* is an encoding of a 64-bit numeric value.
+A *base-62-number* is an encoding of a numeric value.
 It uses ASCII numbers and lowercase and uppercase letters.
 The value is terminated with the `_` character.
 If the value is 0, then the encoding is the `_` character without any digits.