Ensure primitives are "true", "false", "null", or an RFC 8259 compliant
number. (Still need to add test cases.)

String parsing previously did not differ between strict and non-strict
modes, but was not fully compliant with RFC 8259. RFC 8259 requires that
control characters (code points < 0x20) be escaped. This is now enforced
in strict mode. In addition, non-strict mode now does *no* validations
on string contents, much like primitives in non-strict mode.

Parent links and strict parsing are now the default behavior. New macros
JSMN_LOW_MEMORY and JSMN_NON_STRICT disable these behaviors.

JSMN_PARENT_LINKS still exists, but is defined by default unless
JSMN_LOW_MEMORY is defined.

JSMN_STRICT no longer exists. Instead, we have three new macros:

JSMN_PERMISSIVE_PRIMITIVES - Relaxes validation of primitives. Any
characters except whitespace and {}[],:" become allowed. (Normally, only
"true", "false", "null", and RFC 8259 numbers are permitted.)

JSMN_PERMISSIVE_STRINGS - Relaxes validation of strings. Any characters
allowed. (Normally, control characters (<0x20) and invalid escape
sequences are foridden.)

JSMN_PRIMITIVE_KEYS - Allows primitives to be used as object keys.

These can be defined individually, or defining JSMN_NON_STRICT will
cause all to be defined.

Tests have not yet been adapted for these changes.