Remove redundant APIs and replace usages of Encoding module

[ldclakmal]

Purpose

This PR removes the redundant APIs of ballerina/encoding module, with the newly introduced APIs of langlib/array and langlib/string.

Newly introduced APIs in langlib/array module

public function toBase64(byte[] arr) returns string = external;
public function fromBase64(string str) returns byte[]|error = external;
public function toBase16(byte[] arr) returns string = external;
public function fromBase16(string str) returns byte[]|error = external;

Newly introduced APIs in langlib/string module

public function fromBytes(byte[] bytes) returns string|error = external;
public function toBytes(string str) returns byte[] = external;

Matching APIs in the ballerina/encoding module

public function encodeBase64(byte[] input) returns string = external;
public function decodeBase64(string input) returns byte[]|Error = external;
public function encodeHex(byte[] input) returns string = external;
public function decodeHex(string input) returns byte[]|Error = external;

public function byteArrayToString(byte[] content, string encoding = "utf-8") returns string = external;

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Additionally, the following URI encoding/decoding APIs which are in the ballerina/http module is moved to the ballerina/encoding module as follows:

Existing APIs in the ballerina/http module:

public function encode(string url, string charset) returns string|ClientError = external;
public function decode(string url, string charset) returns string|ClientError = external;

Proposed new APIs for the ballerina/encoding module:

public function encodeUriComponent(string uriComponent, string charset) returns string|Error = external;
public function decodeUriComponent(string uriComponent, string charset) returns string|Error = external;

Related discussion: https://groups.google.com/d/msg/ballerina-dev/sbASEwIl44k/0YlP3IcXFwAJ

Fixes ballerina-platform/ballerina-spec#314

Check List

• Read the Contributing Guide
• Required Balo version update
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov-io]

Codecov Report

Merging #18309 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #18309   +/-   ##
=======================================
  Coverage   14.86%   14.86%           
=======================================
  Files          48       48           
  Lines        1265     1265           
  Branches      201      201           
=======================================
  Hits          188      188           
  Misses       1063     1063           
  Partials       14       14

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[wggihan]

Suggested change

	* Copyright (c) 2017, WSO2 Inc. (http://www.wso2.org) All Rights Reserved.
	* Copyright (c) 2019, WSO2 Inc. (http://www.wso2.org) All Rights Reserved.

[ldclakmal]

This is an old class [1]. I just moved it to the encoding module, renamed and updated.

[1] https://github.com/ballerina-platform/ballerina-lang/blob/master/stdlib/http/src/main/java/org/ballerinalang/net/uri/nativeimpl/Encode.java