Support if/else within if condition in condval!

[nvzqz]

The following Rust statement works:

let x = if if true { true } else { false } { "a" } else { "b" };

But the same wrapped in condval! does not parse:

error: expected `{`, found `}`
 --> src/main.rs:4:13
  |
4 |     let x = condval!(if if true { true } else { false } { "a" } else { "b" });
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{`
  |
note: the `if` expression is missing a block after this condition
 --> src/main.rs:4:28
  |
4 |     let x = condval!(if if true { true } else { false } { "a" } else { "b" });
  |                            ^^^^
  = note: this error originates in the macro `$crate::__condval_parser` which comes from the expansion of the macro `condval` (in Nightly builds, run with -Z macro-backtrace for more info)

error: no rules expected the token `{`
   --> src/main.rs:4:47
    |
4   |     let x = condval!(if if true { true } else { false } { "a" } else { "b" });
    |                                               ^ no rules expected this token in macro call
    |
note: while trying to match `if`
   --> condtype/src/lib.rs:212:6
    |
212 |     (if let $pat:pat = $input:block $then:block else $else:block) => {
    |      ^^

error: could not compile `condtype` (bin "condtype") due to 2 previous errors