Errors due to JEP 254: Compact Strings

[gayanW]

Building jpf-core against Java 9 or later would result in following errors due to the implementation changes made to String and related classes in JEP 254

[javac] src/classes/modules/java.base/java/lang/String.java:132: error: incompatible types: Result cannot be converted to char[]
[javac] 		this.value =  StringCoding.decode(cset, x, offset, length);
[javac] 		                                 ^

[javac] src/classes/modules/java.base/java/lang/String.java:160: error: incompatible types: byte[] cannot be converted to char[]
[javac] 			this.value = Arrays.copyOf(x.getValue(), x.length());
[javac] 			                          ^

[javac] src/classes/modules/java.base/java/lang/String.java:166: error: incompatible types: byte[] cannot be converted to char[]
[javac] 		this.value = Arrays.copyOf(x.getValue(), x.length());
[javac] 		                          ^

[javac] src/classes/modules/java.base/java/lang/String.java:205: error: no suitable method found for encode(Charset,char[],int,int)
[javac] 		return StringCoding.encode(x, value, 0, value.length);
[javac] 		                   ^
[javac]     method StringCoding.encode(String,byte,byte[]) is not applicable
[javac]       (actual and formal argument lists differ in length)
[javac]     method StringCoding.encode(Charset,byte,byte[]) is not applicable
[javac]       (actual and formal argument lists differ in length)
[javac]     method StringCoding.encode(byte,byte[]) is not applicable
[javac]       (actual and formal argument lists differ in length)

[javac] src/classes/modules/java.base/java/lang/String.java:236: error: incompatible types: byte[] cannot be converted to char[]
[javac]       return equals0( value,  ((AbstractStringBuilder) charSequence).getValue(), value.length);
[javac]                                                                              ^

https://travis-ci.org/javapathfinder/jpf-core/builds/375815945#L2770-L2790