Reduce collision potential for macros

[OpenByteDev]

I encountered a name collision when using thie macros from this crate together with iced-x86:

use iced_x86::code_asm::registers::gpr8::ch;
fn main() {
    widestring::u16cstr!("test");
}

which results in the following errors

error[E0308]: mismatched types
  --> src\lib.rs:37:5
   |
37 |     widestring::u16cstr!("test");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |     |
   |     expected `u32`, found struct `AsmRegister8`
   |     this expression has type `std::option::Option<(u32, &[u8])>`
   |
   = note: this error originates in the macro `widestring::u16cstr` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0369]: no implementation for `AsmRegister8 & {integer}`  --> src\lib.rs:37:5
   |
37 |     widestring::u16cstr!("test");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |     |
   |     AsmRegister8
   |     {integer}
   |
   = note: this error originates in the macro `widestring::u16cstr` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0369]: cannot subtract `{integer}` from `AsmRegister8` 
  --> src\lib.rs:37:5
   |
37 |     widestring::u16cstr!("test");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |     |
   |     AsmRegister8
   |     {integer}
   |
   = note: this error originates in the macro `widestring::u16cstr` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0605]: non-primitive cast: `AsmRegister8` as `u16`     
  --> src\lib.rs:37:5
   |
37 |     widestring::u16cstr!("test");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
   |
   = note: this error originates in the macro `widestring::u16cstr` (in Nightly builds, run with -Z macro-backtrace for more info)

This pull request adds the prefix _widestring_ to all locals declared in the macros and also uses fully qualified names for one occurance of u16/u32 and Some.