Fix boolean parsing and make SimpleTypeSerializer and SimpleTypeDeserializer public

[Mingun]

This PR makes SimpleTypeSerializer and SimpleTypeDeserializer public, as requested in #824. It also includes two other changes.

When checking documentation for the SimpleTypeDeserializer I noticed, that we parse more boolean representations than described in the XML Schema specification. I do not see much reasons to keep that additional variants anymore, also because of the next implemented change. If you need more representations, you can use the wrapper type or #[serde(deserialize_with)] attribute.

The last change that this PR contains is final transfer of the responsibility of generating errors about mismatched types to the type being deserialized. That means that we no more need DeError::InvalidInt, DeError::InvalidFloat and DeError::InvalidBoolean errors, so their are removed. The changes can be notified in this code snippet:

#[test]
fn integer() {
    quick_xml::de::from_str::<usize>("string").unwrap();
}

#[test]
fn float() {
    quick_xml::de::from_str::<f64>("string").unwrap();
}

#[test]
fn boolean() {
    quick_xml::de::from_str::<bool>("trues").unwrap();
}

Before:

---- temp::integer stdout ----
thread 'temp::integer' panicked at tests/serde-issues.rs:560:48:
called `Result::unwrap()` on an `Err` value: InvalidInt(ParseIntError { kind: InvalidDigit })

---- temp::float stdout ----
thread 'temp::float' panicked at tests/serde-issues.rs:565:46:
called `Result::unwrap()` on an `Err` value: InvalidFloat(ParseFloatError { kind: Invalid })

---- temp::boolean stdout ----
thread 'temp::boolean' panicked at tests/serde-issues.rs:570:46:
called `Result::unwrap()` on an `Err` value: InvalidBoolean("trues")

After:

---- temp::integer stdout ----
thread 'temp::integer' panicked at tests/serde-issues.rs:560:48:
called `Result::unwrap()` on an `Err` value: Custom("invalid type: string \"string\", expected usize")

---- temp::float stdout ----
thread 'temp::float' panicked at tests/serde-issues.rs:565:46:
called `Result::unwrap()` on an `Err` value: Custom("invalid type: string \"string\", expected f64")

---- temp::boolean stdout ----
thread 'temp::boolean' panicked at tests/serde-issues.rs:570:46:
called `Result::unwrap()` on an `Err` value: Custom("invalid type: string \"trues\", expected a boolean")

Closes #824

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Codecov Report

Attention: Patch coverage is 69.09091% with 34 lines in your changes missing coverage. Please review.

Project coverage is 60.05%. Comparing base (a9391f3) to head (83d2957).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/de/simple_type.rs	71.18%	17 Missing ⚠️
src/de/mod.rs	44.44%	15 Missing ⚠️
src/de/key.rs	75.00%	1 Missing ⚠️
src/utils.rs	95.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #826      +/-   ##
==========================================
- Coverage   60.21%   60.05%   -0.16%     
==========================================
  Files          41       41              
  Lines       16021    16031      +10     
==========================================
- Hits         9647     9628      -19     
- Misses       6374     6403      +29     

Flag	Coverage Δ
unittests	60.05% <69.09%> (-0.16%)	⬇️

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